[Light Switch] Follow Night Light mode (#43683)

<!-- Enter a brief description/summary of your PR here. What does it
fix/what does it change/how was it tested (even manually, if necessary)?
-->
## Summary of the Pull Request
Introduces a new mode that will have Light Switch follow Windows Night
Light.

<!-- Please review the items on the PR checklist before submitting-->
## PR Checklist

- [x] Closes: https://github.com/microsoft/PowerToys/issues/42457
- [x] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **Tests:** Added/updated and all pass
- [x] **Localization:** All end-user-facing strings can be localized
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

<!-- Provide a more detailed description of the PR, other things fixed,
or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments
Strictly follows the state of Night Light. When NL is on, LS will be
switch to dark mode and when NL is off, LS will switch to light mode
(with respect to the users system/app selection).

<!-- Describe how you validated the behavior. Add automated tests
wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed
Turn on Follow Night Light mode
Change night light!

## Notes

.github/actions/spell-check/expect.txt

@@ -144,6 +144,8 @@ BLENDFUNCTION
 blittable
 Blockquotes
 blt
+bluelightreduction
+bluelightreductionstate
 BLURBEHIND
 BLURREGION
 bmi
@@ -1115,6 +1117,7 @@ NEWPLUSSHELLEXTENSIONWIN
 newrow
 nicksnettravels
 NIF
+nightlight
 NLog
 NLSTEXT
 NMAKE

src/modules/LightSwitch/LightSwitchModuleInterface/dllmain.cpp

@@ -50,6 +50,7 @@ enum class ScheduleMode
     Off,
     FixedHours,
     SunsetToSunrise,
+    FollowNightLight,
     // add more later
 };
 
@@ -61,6 +62,8 @@ inline std::wstring ToString(ScheduleMode mode)
         return L"SunsetToSunrise";
     case ScheduleMode::FixedHours:
         return L"FixedHours";
+    case ScheduleMode::FollowNightLight:
+        return L"FollowNightLight";
     default:
         return L"Off";
     }
@@ -72,6 +75,8 @@ inline ScheduleMode FromString(const std::wstring& str)
         return ScheduleMode::SunsetToSunrise;
     if (str == L"FixedHours")
         return ScheduleMode::FixedHours;
+    if (str == L"FollowNightLight")
+        return ScheduleMode::FollowNightLight;
     return ScheduleMode::Off;
 }
 
@@ -167,7 +172,9 @@ public:
             ToString(g_settings.m_scheduleMode),
             { { L"Off", L"Disable the schedule" },
               { L"FixedHours", L"Set hours manually" },
-              { L"SunsetToSunrise", L"Use sunrise/sunset times" } });
+              { L"SunsetToSunrise", L"Use sunrise/sunset times" },
+              { L"FollowNightLight", L"Follow Windows Night Light state" }
+            });
 
         // Integer spinners
         settings.add_int_spinner(

src/modules/LightSwitch/LightSwitchService/LightSwitchService.cpp

@@ -13,10 +13,12 @@
 #include <utils/logger_helper.h>
 #include "LightSwitchStateManager.h"
 #include <LightSwitchUtils.h>
+#include <NightLightRegistryObserver.h>
 
 SERVICE_STATUS g_ServiceStatus = {};
 SERVICE_STATUS_HANDLE g_StatusHandle = nullptr;
 HANDLE g_ServiceStopEvent = nullptr;
+static LightSwitchStateManager* g_stateManagerPtr = nullptr;
 
 VOID WINAPI ServiceMain(DWORD argc, LPTSTR* argv);
 VOID WINAPI ServiceCtrlHandler(DWORD dwCtrl);
@@ -168,7 +170,15 @@ static void DetectAndHandleExternalThemeChange(LightSwitchStateManager& stateMan
     }
 
     // Use shared helper (handles wraparound logic)
-    bool shouldBeLight = ShouldBeLight(nowMinutes, effectiveLight, effectiveDark);
+    bool shouldBeLight = false;
+    if (s.scheduleMode == ScheduleMode::FollowNightLight)
+    {
+        shouldBeLight = !IsNightLightEnabled();
+    } 
+    else
+    {
+        shouldBeLight = ShouldBeLight(nowMinutes, effectiveLight, effectiveDark);
+    }
 
     // Compare current system/apps theme
     bool currentSystemLight = GetCurrentSystemTheme();
@@ -199,15 +209,40 @@ DWORD WINAPI ServiceWorkerThread(LPVOID lpParam)
     // Initialization
     // ────────────────────────────────────────────────────────────────
     static LightSwitchStateManager stateManager;
+    g_stateManagerPtr = &stateManager;
 
     LightSwitchSettings::instance().InitFileWatcher();
 
     HANDLE hManualOverride = OpenEventW(SYNCHRONIZE | EVENT_MODIFY_STATE, FALSE, L"POWERTOYS_LIGHTSWITCH_MANUAL_OVERRIDE");
     HANDLE hSettingsChanged = LightSwitchSettings::instance().GetSettingsChangedEvent();
 
+    static std::unique_ptr<NightLightRegistryObserver> g_nightLightWatcher;
+
     LightSwitchSettings::instance().LoadSettings();
     const auto& settings = LightSwitchSettings::instance().settings();
 
+    // after loading settings:
+    bool nightLightNeeded = (settings.scheduleMode == ScheduleMode::FollowNightLight);
+
+    if (nightLightNeeded && !g_nightLightWatcher)
+    {
+        Logger::info(L"[LightSwitchService] Starting Night Light registry watcher...");
+
+        g_nightLightWatcher = std::make_unique<NightLightRegistryObserver>(
+            HKEY_CURRENT_USER,
+            NIGHT_LIGHT_REGISTRY_PATH,
+            []() {
+                if (g_stateManagerPtr)
+                    g_stateManagerPtr->OnNightLightChange();
+            });
+    }
+    else if (!nightLightNeeded && g_nightLightWatcher)
+    {
+        Logger::info(L"[LightSwitchService] Stopping Night Light registry watcher...");
+        g_nightLightWatcher->Stop();
+        g_nightLightWatcher.reset();
+    }
+
     SYSTEMTIME st;
     GetLocalTime(&st);
     int nowMinutes = st.wHour * 60 + st.wMinute;
@@ -274,6 +309,31 @@ DWORD WINAPI ServiceWorkerThread(LPVOID lpParam)
             ResetEvent(hSettingsChanged);
             LightSwitchSettings::instance().LoadSettings();
             stateManager.OnSettingsChanged();
+
+            const auto& settings = LightSwitchSettings::instance().settings();
+            bool nightLightNeeded = (settings.scheduleMode == ScheduleMode::FollowNightLight);
+
+            if (nightLightNeeded && !g_nightLightWatcher)
+            {
+                Logger::info(L"[LightSwitchService] Starting Night Light registry watcher...");
+
+                g_nightLightWatcher = std::make_unique<NightLightRegistryObserver>(
+                    HKEY_CURRENT_USER,
+                    NIGHT_LIGHT_REGISTRY_PATH,
+                    []() {
+                        if (g_stateManagerPtr)
+                            g_stateManagerPtr->OnNightLightChange();
+                    });
+
+                stateManager.OnNightLightChange();
+            }
+            else if (!nightLightNeeded && g_nightLightWatcher)
+            {
+                Logger::info(L"[LightSwitchService] Stopping Night Light registry watcher...");
+                g_nightLightWatcher->Stop();
+                g_nightLightWatcher.reset();
+            }
+
             continue;
         }
     }
@@ -285,6 +345,11 @@ DWORD WINAPI ServiceWorkerThread(LPVOID lpParam)
         CloseHandle(hManualOverride);
     if (hParent)
         CloseHandle(hParent);
+    if (g_nightLightWatcher)
+    {
+        g_nightLightWatcher->Stop();
+        g_nightLightWatcher.reset();
+    }
 
     Logger::info(L"[LightSwitchService] Worker thread exiting cleanly.");
     return 0;

src/modules/LightSwitch/LightSwitchService/LightSwitchService.vcxproj

@@ -76,6 +76,7 @@
     <ClCompile Include="LightSwitchService.cpp" />
     <ClCompile Include="LightSwitchSettings.cpp" />
     <ClCompile Include="LightSwitchStateManager.cpp" />
+    <ClCompile Include="NightLightRegistryObserver.cpp" />
     <ClCompile Include="SettingsConstants.cpp" />
     <ClCompile Include="ThemeHelper.cpp" />
     <ClCompile Include="ThemeScheduler.cpp" />
@@ -88,6 +89,7 @@
     <ClInclude Include="LightSwitchSettings.h" />
     <ClInclude Include="LightSwitchStateManager.h" />
     <ClInclude Include="LightSwitchUtils.h" />
+    <ClInclude Include="NightLightRegistryObserver.h" />
     <ClInclude Include="SettingsConstants.h" />
     <ClInclude Include="SettingsObserver.h" />
     <ClInclude Include="ThemeHelper.h" />

src/modules/LightSwitch/LightSwitchService/LightSwitchService.vcxproj.filters

@@ -36,6 +36,9 @@
     <ClCompile Include="LightSwitchStateManager.cpp">
       <Filter>Source Files</Filter>
     </ClCompile>
+    <ClCompile Include="NightLightRegistryObserver.cpp">
+      <Filter>Source Files</Filter>
+    </ClCompile>
   </ItemGroup>
   <ItemGroup>
     <ClInclude Include="ThemeScheduler.h">
@@ -62,6 +65,9 @@
     <ClInclude Include="LightSwitchUtils.h">
       <Filter>Header Files</Filter>
     </ClInclude>
+    <ClInclude Include="NightLightRegistryObserver.h">
+      <Filter>Header Files</Filter>
+    </ClInclude>
   </ItemGroup>
   <ItemGroup>
     <Natvis Include="$(MSBuildThisFileDirectory)..\..\natvis\wil.natvis" />

src/modules/LightSwitch/LightSwitchService/LightSwitchSettings.h

@@ -19,7 +19,8 @@ enum class ScheduleMode
 {
     Off,
     FixedHours,
-    SunsetToSunrise
+    SunsetToSunrise,
+    FollowNightLight,
     // Add more in the future
 };
 
@@ -31,6 +32,8 @@ inline std::wstring ToString(ScheduleMode mode)
         return L"FixedHours";
     case ScheduleMode::SunsetToSunrise:
         return L"SunsetToSunrise";
+    case ScheduleMode::FollowNightLight:
+        return L"FollowNightLight";
     default:
         return L"Off";
     }
@@ -42,6 +45,8 @@ inline ScheduleMode FromString(const std::wstring& str)
         return ScheduleMode::SunsetToSunrise;
     if (str == L"FixedHours")
         return ScheduleMode::FixedHours;
+    if (str == L"FollowNightLight")
+        return ScheduleMode::FollowNightLight;
     else
         return ScheduleMode::Off;
 }

src/modules/LightSwitch/LightSwitchService/LightSwitchStateManager.cpp

@@ -31,7 +31,10 @@ void LightSwitchStateManager::OnSettingsChanged()
 void LightSwitchStateManager::OnTick(int currentMinutes)
 {
     std::lock_guard<std::mutex> lock(_stateMutex);
-    EvaluateAndApplyIfNeeded();
+    if (_state.lastAppliedMode != ScheduleMode::FollowNightLight)
+    {
+        EvaluateAndApplyIfNeeded();
+    }
 }
 
 // Called when manual override is triggered
@@ -49,8 +52,38 @@ void LightSwitchStateManager::OnManualOverride()
         _state.isAppsLightActive = GetCurrentAppsTheme();
 
         Logger::debug(L"[LightSwitchStateManager] Synced internal theme state to current system theme ({}) and apps theme ({}).",
-                     (_state.isSystemLightActive ? L"light" : L"dark"),
-                     (_state.isAppsLightActive ? L"light" : L"dark"));
+                      (_state.isSystemLightActive ? L"light" : L"dark"),
+                      (_state.isAppsLightActive ? L"light" : L"dark"));
+    }
+
+    EvaluateAndApplyIfNeeded();
+}
+
+// Runs with the registry observer detects a change in Night Light settings.
+void LightSwitchStateManager::OnNightLightChange()
+{
+    std::lock_guard<std::mutex> lock(_stateMutex);
+
+    bool newNightLightState = IsNightLightEnabled();
+
+    // In Follow Night Light mode, treat a Night Light toggle as a boundary
+    if (_state.lastAppliedMode == ScheduleMode::FollowNightLight && _state.isManualOverride)
+    {
+        Logger::info(L"[LightSwitchStateManager] Night Light changed while manual override active; "
+                     L"treating as a boundary and clearing manual override.");
+        _state.isManualOverride = false;
+    }
+
+    if (newNightLightState != _state.isNightLightActive)
+    {
+        Logger::info(L"[LightSwitchStateManager] Night Light toggled to {}",
+                     newNightLightState ? L"ON" : L"OFF");
+
+        _state.isNightLightActive = newNightLightState;
+    }
+    else
+    {
+        Logger::debug(L"[LightSwitchStateManager] Night Light change event fired, but no actual change.");
     }
 
     EvaluateAndApplyIfNeeded();
@@ -77,9 +110,9 @@ void LightSwitchStateManager::SyncInitialThemeState()
     _state.isSystemLightActive = GetCurrentSystemTheme();
     _state.isAppsLightActive = GetCurrentAppsTheme();
     Logger::debug(L"[LightSwitchStateManager] Synced initial state to current system theme ({})",
-                 _state.isSystemLightActive ? L"light" : L"dark");
+                  _state.isSystemLightActive ? L"light" : L"dark");
     Logger::debug(L"[LightSwitchStateManager] Synced initial state to current apps theme ({})",
-                 _state.isAppsLightActive ? L"light" : L"dark");
+                  _state.isAppsLightActive ? L"light" : L"dark");
 }
 
 static std::pair<int, int> update_sun_times(auto& settings)
@@ -194,7 +227,15 @@ void LightSwitchStateManager::EvaluateAndApplyIfNeeded()
 
     _state.lastAppliedMode = _currentSettings.scheduleMode;
 
-    bool shouldBeLight = ShouldBeLight(now, _state.effectiveLightMinutes, _state.effectiveDarkMinutes);
+    bool shouldBeLight = false;
+    if (_currentSettings.scheduleMode == ScheduleMode::FollowNightLight)
+    {
+        shouldBeLight = !_state.isNightLightActive;
+    }
+    else
+    {
+        shouldBeLight = ShouldBeLight(now, _state.effectiveLightMinutes, _state.effectiveDarkMinutes);
+    }
 
     bool appsNeedsToChange = _currentSettings.changeApps && (_state.isAppsLightActive != shouldBeLight);
     bool systemNeedsToChange = _currentSettings.changeSystem && (_state.isSystemLightActive != shouldBeLight);
@@ -227,6 +268,3 @@ void LightSwitchStateManager::EvaluateAndApplyIfNeeded()
 
     _state.lastTickMinutes = now;
 }
-
-
-

src/modules/LightSwitch/LightSwitchService/LightSwitchStateManager.h

@@ -9,6 +9,7 @@ struct LightSwitchState
     bool isManualOverride = false;
     bool isSystemLightActive = false;
     bool isAppsLightActive = false;
+    bool isNightLightActive = false;
     int lastEvaluatedDay = -1;
     int lastTickMinutes = -1;
 
@@ -32,6 +33,9 @@ public:
     // Called when manual override is toggled (via shortcut or system change).
     void OnManualOverride();
 
+    // Called when night light changes in windows settings
+    void OnNightLightChange();
+
     // Initial sync at startup to align internal state with system theme
     void SyncInitialThemeState();
 

src/modules/LightSwitch/LightSwitchService/NightLightRegistryObserver.cpp

@@ -0,0 +1 @@
+#include "NightLightRegistryObserver.h"

src/modules/LightSwitch/LightSwitchService/NightLightRegistryObserver.h

@@ -0,0 +1,134 @@
+#pragma once
+#include <wtypes.h>
+#include <string>
+#include <functional>
+#include <thread>
+#include <atomic>
+#include <mutex>
+
+class NightLightRegistryObserver
+{
+public:
+    NightLightRegistryObserver(HKEY root, const std::wstring& subkey, std::function<void()> callback) :
+        _root(root), _subkey(subkey), _callback(std::move(callback)), _stop(false)
+    {
+        _thread = std::thread([this]() { this->Run(); });
+    }
+
+    ~NightLightRegistryObserver()
+    {
+        Stop();
+    }
+
+    void Stop()
+    {
+        _stop = true;
+
+        {
+            std::lock_guard<std::mutex> lock(_mutex);
+            if (_event)
+                SetEvent(_event);
+        }
+
+        if (_thread.joinable())
+            _thread.join();
+
+        std::lock_guard<std::mutex> lock(_mutex);
+        if (_hKey)
+        {
+            RegCloseKey(_hKey);
+            _hKey = nullptr;
+        }
+
+        if (_event)
+        {
+            CloseHandle(_event);
+            _event = nullptr;
+        }
+    }
+
+
+private:
+    void Run()
+    {
+        {
+            std::lock_guard<std::mutex> lock(_mutex);
+            if (RegOpenKeyExW(_root, _subkey.c_str(), 0, KEY_NOTIFY, &_hKey) != ERROR_SUCCESS)
+                return;
+
+            _event = CreateEventW(nullptr, TRUE, FALSE, nullptr);
+            if (!_event)
+            {
+                RegCloseKey(_hKey);
+                _hKey = nullptr;
+                return;
+            }
+        }
+
+        while (!_stop)
+        {
+            HKEY hKeyLocal = nullptr;
+            HANDLE eventLocal = nullptr;
+
+            {
+                std::lock_guard<std::mutex> lock(_mutex);
+                if (_stop)
+                    break;
+
+                hKeyLocal = _hKey;
+                eventLocal = _event;
+            }
+
+            if (!hKeyLocal || !eventLocal)
+                break;
+
+            if (_stop)
+                break;
+
+            if (RegNotifyChangeKeyValue(hKeyLocal, FALSE, REG_NOTIFY_CHANGE_LAST_SET, eventLocal, TRUE) != ERROR_SUCCESS)
+                break;
+
+            DWORD wait = WaitForSingleObject(eventLocal, INFINITE);
+            if (_stop || wait == WAIT_FAILED)
+                break;
+
+            ResetEvent(eventLocal);
+
+            if (!_stop && _callback)
+            {
+                try
+                {
+                    _callback();
+                }
+                catch (...)
+                {
+                }
+            }
+        }
+
+        {
+            std::lock_guard<std::mutex> lock(_mutex);
+            if (_hKey)
+            {
+                RegCloseKey(_hKey);
+                _hKey = nullptr;
+            }
+
+            if (_event)
+            {
+                CloseHandle(_event);
+                _event = nullptr;
+            }
+        }
+    }
+
+
+    HKEY _root;
+    std::wstring _subkey;
+    std::function<void()> _callback;
+    HANDLE _event = nullptr;
+    HKEY _hKey = nullptr;
+    std::thread _thread;
+    std::atomic<bool> _stop;
+    std::mutex _mutex;
+};

src/modules/LightSwitch/LightSwitchService/SettingsConstants.h

@@ -11,4 +11,7 @@ enum class SettingId
     Sunset_Offset,
     ChangeSystem,
     ChangeApps
-};
+};
+
+constexpr wchar_t PERSONALIZATION_REGISTRY_PATH[] = L"Software\\Microsoft\\Windows\\CurrentVersion\\Themes\\Personalize";
+constexpr wchar_t NIGHT_LIGHT_REGISTRY_PATH[] = L"Software\\Microsoft\\Windows\\CurrentVersion\\CloudStore\\Store\\DefaultAccount\\Current\\default$windows.data.bluelightreduction.bluelightreductionstate\\windows.data.bluelightreduction.bluelightreductionstate";

src/modules/LightSwitch/LightSwitchService/ThemeHelper.cpp

@@ -3,6 +3,7 @@
 #include <logger/logger.h>
 #include <utils/logger_helper.h>
 #include "ThemeHelper.h"
+#include <SettingsConstants.h>
 
 // Controls changing the themes.
 
@@ -10,7 +11,7 @@ static void ResetColorPrevalence()
 {
     HKEY hKey;
     if (RegOpenKeyEx(HKEY_CURRENT_USER,
-                     L"Software\\Microsoft\\Windows\\CurrentVersion\\Themes\\Personalize",
+                     PERSONALIZATION_REGISTRY_PATH,
                      0,
                      KEY_SET_VALUE,
                      &hKey) == ERROR_SUCCESS)
@@ -31,7 +32,7 @@ void SetAppsTheme(bool mode)
 {
     HKEY hKey;
     if (RegOpenKeyEx(HKEY_CURRENT_USER,
-                     L"Software\\Microsoft\\Windows\\CurrentVersion\\Themes\\Personalize",
+                     PERSONALIZATION_REGISTRY_PATH,
                      0,
                      KEY_SET_VALUE,
                      &hKey) == ERROR_SUCCESS)
@@ -50,7 +51,7 @@ void SetSystemTheme(bool mode)
 {
     HKEY hKey;
     if (RegOpenKeyEx(HKEY_CURRENT_USER,
-                     L"Software\\Microsoft\\Windows\\CurrentVersion\\Themes\\Personalize",
+                     PERSONALIZATION_REGISTRY_PATH,
                      0,
                      KEY_SET_VALUE,
                      &hKey) == ERROR_SUCCESS)
@@ -79,7 +80,7 @@ bool GetCurrentSystemTheme()
     DWORD size = sizeof(value);
 
     if (RegOpenKeyEx(HKEY_CURRENT_USER,
-                     L"Software\\Microsoft\\Windows\\CurrentVersion\\Themes\\Personalize",
+                     PERSONALIZATION_REGISTRY_PATH,
                      0,
                      KEY_READ,
                      &hKey) == ERROR_SUCCESS)
@@ -98,7 +99,7 @@ bool GetCurrentAppsTheme()
     DWORD size = sizeof(value);
 
     if (RegOpenKeyEx(HKEY_CURRENT_USER,
-                     L"Software\\Microsoft\\Windows\\CurrentVersion\\Themes\\Personalize",
+                     PERSONALIZATION_REGISTRY_PATH,
                      0,
                      KEY_READ,
                      &hKey) == ERROR_SUCCESS)
@@ -109,3 +110,30 @@ bool GetCurrentAppsTheme()
 
     return value == 1; // true = light, false = dark
 }
+
+bool IsNightLightEnabled()
+{
+    HKEY hKey;
+    const wchar_t* path = NIGHT_LIGHT_REGISTRY_PATH;
+
+    if (RegOpenKeyExW(HKEY_CURRENT_USER, path, 0, KEY_READ, &hKey) != ERROR_SUCCESS)
+        return false;
+
+    // RegGetValueW will set size to the size of the data and we expect that to be at least 25 bytes (we need to access bytes 23 and 24)
+    DWORD size = 0;
+    if (RegGetValueW(hKey, nullptr, L"Data", RRF_RT_REG_BINARY, nullptr, nullptr, &size) != ERROR_SUCCESS || size < 25)
+    {
+        RegCloseKey(hKey);
+        return false;
+    }
+
+    std::vector<BYTE> data(size);
+    if (RegGetValueW(hKey, nullptr, L"Data", RRF_RT_REG_BINARY, nullptr, data.data(), &size) != ERROR_SUCCESS)
+    {
+        RegCloseKey(hKey);
+        return false;
+    }
+
+    RegCloseKey(hKey);
+    return data[23] == 0x10 && data[24] == 0x00;
+}

src/modules/LightSwitch/LightSwitchService/ThemeHelper.h

@@ -3,3 +3,4 @@ void SetSystemTheme(bool dark);
 void SetAppsTheme(bool dark);
 bool GetCurrentSystemTheme();
 bool GetCurrentAppsTheme();
+bool IsNightLightEnabled();

src/settings-ui/Settings.UI/Helpers/StartProcessHelper.cs

@@ -11,6 +11,7 @@ namespace Microsoft.PowerToys.Settings.UI.Helpers
     {
         public const string ColorsSettings = "ms-settings:colors";
         public const string DiagnosticsAndFeedback = "ms-settings:privacy-feedback";
+        public const string NightLightSettings = "ms-settings:nightlight";
 
         public static string AnimationsSettings => OSVersionHelper.IsWindows11()
             ? "ms-settings:easeofaccess-visualeffects"

src/settings-ui/Settings.UI/SettingsXAML/Views/LightSwitchPage.xaml

@@ -67,6 +67,10 @@
                                     x:Uid="LightSwitch_ModeSunsetToSunrise"
                                     AutomationProperties.AutomationId="SunCBItem_LightSwitch"
                                     Tag="SunsetToSunrise" />
+                                <ComboBoxItem
+                                    x:Uid="LightSwitch_ModeFollowNightLight"
+                                    AutomationProperties.AutomationId="FollowNightLightCBItem_LightSwitch"
+                                    Tag="FollowNightLight" />
                             </ComboBox>
                             <tkcontrols:SettingsExpander.Items>
                                 <tkcontrols:SettingsCard
@@ -152,6 +156,33 @@
                                         IsOpen="True"
                                         Severity="Informational" />
                                 </tkcontrols:SettingsCard>
+                                <tkcontrols:SettingsCard
+                                    x:Name="FollowNightLightCard"
+                                    Padding="0"
+                                    HorizontalContentAlignment="Stretch"
+                                    Background="{ThemeResource InfoBarInformationalSeverityBackgroundBrush}"
+                                    ContentAlignment="Vertical"
+                                    Visibility="Collapsed">
+                                    <InfoBar
+                                        Background="Transparent"
+                                        BorderThickness="0"
+                                        IsClosable="False"
+                                        IsOpen="True"
+                                        Severity="Informational">
+                                        <StackPanel VerticalAlignment="Center" Orientation="Horizontal">
+                                            <TextBlock VerticalAlignment="Center">
+                                                <Run x:Uid="LightSwitch_FollowNightLightCardMessage" />
+                                            </TextBlock>
+
+                                            <HyperlinkButton
+                                                x:Uid="LightSwitch_NightLightSettingsButton"
+                                                Margin="3,0,0,0"
+                                                Padding="0"
+                                                Click="OpenNightLightSettings_Click" />
+                                        </StackPanel>
+
+                                    </InfoBar>
+                                </tkcontrols:SettingsCard>
                             </tkcontrols:SettingsExpander.Items>
                         </tkcontrols:SettingsExpander>
                         <InfoBar
@@ -350,13 +381,25 @@
                         <Setter Target="SunLocation_Card.Visibility" Value="Visible" />
                         <Setter Target="SunOffset_Card.Visibility" Value="Visible" />
                         <Setter Target="NoScheduleCard.Visibility" Value="Collapsed" />
+                        <Setter Target="FollowNightLightCard.Visibility" Value="Collapsed" />
                     </VisualState.Setters>
                 </VisualState>
+
                 <VisualState x:Name="ManualState">
                     <VisualState.Setters>
                         <Setter Target="Fixed_TurnOnCard.Visibility" Value="Visible" />
                         <Setter Target="Fixed_TurnOffCard.Visibility" Value="Visible" />
                         <Setter Target="NoScheduleCard.Visibility" Value="Collapsed" />
+                        <Setter Target="FollowNightLightCard.Visibility" Value="Collapsed" />
+                    </VisualState.Setters>
+                </VisualState>
+
+                <VisualState x:Name="FollowNightLightState">
+                    <VisualState.Setters>
+                        <Setter Target="Fixed_TurnOnCard.Visibility" Value="Collapsed" />
+                        <Setter Target="Fixed_TurnOffCard.Visibility" Value="Collapsed" />
+                        <Setter Target="NoScheduleCard.Visibility" Value="Collapsed" />
+                        <Setter Target="FollowNightLightCard.Visibility" Value="Visible" />
                     </VisualState.Setters>
                 </VisualState>
             </VisualStateGroup>

src/settings-ui/Settings.UI/SettingsXAML/Views/LightSwitchPage.xaml.cs

@@ -355,6 +355,10 @@ namespace Microsoft.PowerToys.Settings.UI.Views
                     VisualStateManager.GoToState(this, "SunsetToSunriseState", true);
                     this.SunriseModeChartState();
                     break;
+                case "FollowNightLight":
+                    VisualStateManager.GoToState(this, "FollowNightLightState", true);
+                    TimelineCard.Visibility = Visibility.Collapsed;
+                    break;
                 default:
                     VisualStateManager.GoToState(this, "OffState", true);
                     this.TimelineCard.Visibility = Visibility.Collapsed;
@@ -362,6 +366,18 @@ namespace Microsoft.PowerToys.Settings.UI.Views
             }
         }
 
+        private void OpenNightLightSettings_Click(object sender, RoutedEventArgs e)
+        {
+            try
+            {
+                Helpers.StartProcessHelper.Start(Helpers.StartProcessHelper.NightLightSettings);
+            }
+            catch (Exception ex)
+            {
+                Logger.LogError("Error while trying to open the system night light settings", ex);
+            }
+        }
+
         private void SunriseModeChartState()
         {
             if (this.ViewModel.Latitude != "0.0" && this.ViewModel.Longitude != "0.0")

src/settings-ui/Settings.UI/Strings/en-us/Resources.resw

@@ -1,17 +1,17 @@
 <?xml version="1.0" encoding="utf-8"?>
 <root>
-  <!--
-    Microsoft ResX Schema
-
+  <!-- 
+    Microsoft ResX Schema 
+    
     Version 2.0
-
-    The primary goals of this format is to allow a simple XML format
-    that is mostly human readable. The generation and parsing of the
-    various data types are done through the TypeConverter classes
+    
+    The primary goals of this format is to allow a simple XML format 
+    that is mostly human readable. The generation and parsing of the 
+    various data types are done through the TypeConverter classes 
     associated with the data types.
-
+    
     Example:
-
+    
     ... ado.net/XML headers & schema ...
     <resheader name="resmimetype">text/microsoft-resx</resheader>
     <resheader name="version">2.0</resheader>
@@ -26,36 +26,36 @@
         <value>[base64 mime encoded string representing a byte array form of the .NET Framework object]</value>
         <comment>This is a comment</comment>
     </data>
-
-    There are any number of "resheader" rows that contain simple
+                
+    There are any number of "resheader" rows that contain simple 
     name/value pairs.
-
-    Each data row contains a name, and value. The row also contains a
-    type or mimetype. Type corresponds to a .NET class that support
-    text/value conversion through the TypeConverter architecture.
-    Classes that don't support this are serialized and stored with the
+    
+    Each data row contains a name, and value. The row also contains a 
+    type or mimetype. Type corresponds to a .NET class that support 
+    text/value conversion through the TypeConverter architecture. 
+    Classes that don't support this are serialized and stored with the 
     mimetype set.
-
-    The mimetype is used for serialized objects, and tells the
-    ResXResourceReader how to depersist the object. This is currently not
+    
+    The mimetype is used for serialized objects, and tells the 
+    ResXResourceReader how to depersist the object. This is currently not 
     extensible. For a given mimetype the value must be set accordingly:
-
-    Note - application/x-microsoft.net.object.binary.base64 is the format
-    that the ResXResourceWriter will generate, however the reader can
+    
+    Note - application/x-microsoft.net.object.binary.base64 is the format 
+    that the ResXResourceWriter will generate, however the reader can 
     read any of the formats listed below.
-
+    
     mimetype: application/x-microsoft.net.object.binary.base64
-    value   : The object must be serialized with
+    value   : The object must be serialized with 
             : System.Runtime.Serialization.Formatters.Binary.BinaryFormatter
             : and then encoded with base64 encoding.
-
+    
     mimetype: application/x-microsoft.net.object.soap.base64
-    value   : The object must be serialized with
+    value   : The object must be serialized with 
             : System.Runtime.Serialization.Formatters.Soap.SoapFormatter
             : and then encoded with base64 encoding.
 
     mimetype: application/x-microsoft.net.object.bytearray.base64
-    value   : The object must be serialized into a byte array
+    value   : The object must be serialized into a byte array 
             : using a System.ComponentModel.TypeConverter
             : and then encoded with base64 encoding.
     -->
@@ -5760,4 +5760,13 @@ To record a specific window, enter the hotkey with the Alt key in the opposite m
     <value>A modern UI built with Fluent Design</value>
     <comment>Fluent Design is a product name, do not loc</comment>
   </data>
-</root>
+  <data name="LightSwitch_ModeFollowNightLight.Content" xml:space="preserve">
+    <value>Follow Night Light</value>
+  </data>
+  <data name="LightSwitch_NightLightSettingsButton.Content" xml:space="preserve">
+    <value>Personalize your Night Light settings.</value>
+  </data>
+  <data name="LightSwitch_FollowNightLightCardMessage.Text" xml:space="preserve">
+    <value>Following Night Light settings.</value>
+  </data>
+</root>

src/settings-ui/Settings.UI/ViewModels/LightSwitchViewModel.cs

@@ -42,6 +42,7 @@ namespace Microsoft.PowerToys.Settings.UI.ViewModels
                 "Off",
                 "FixedHours",
                 "SunsetToSunrise",
+                "FollowNightLight",
             };
 
             _toggleThemeHotkey = _moduleSettings.Properties.ToggleThemeHotkey.Value;