Nightcurtain is a nudge utility that gently encourages sleep by using Windows color filters instead of forcibly blocking the screen like Screen Time on macOS, and is designed to run automatically in conjunction with the Task Scheduler.

nightcurtain.cs

#!/usr/bin/env dotnet
#:sdk Microsoft.NET.Sdk
#:property OutputType=WinExe
#:property TargetFramework=net10.0-windows

// This program can be scheduled using Windows Task Scheduler.
// Below are some example `schtasks` CLI commands to register tasks for `nightcurtain.exe`.
//
// 1. Enable color filter every night at 10:00 PM
// schtasks /Create /TN "NightCurtain_Enable" /TR "C:\Path\To\nightcurtain.exe enable" /SC DAILY /ST 22:00 /F
//
// 2. Disable color filter every morning at 6:00 AM
// schtasks /Create /TN "NightCurtain_Disable" /TR "C:\Path\To\nightcurtain.exe disable" /SC DAILY /ST 06:00 /F
//
// 3. Run automatically on system startup (time-based mode)
// schtasks /Create /TN "NightCurtain_AtStartup" /TR "C:\Path\To\nightcurtain.exe" /SC ONSTART /F
//
// 4. Run automatically when the user logs in (time-based mode)
// schtasks /Create /TN "NightCurtain_AtLogon" /TR "C:\Path\To\nightcurtain.exe" /SC ONLOGON /F
//
// Notes:
// - Replace `C:\Path\To\nightcurtain.exe` with the actual path.
// - When no arguments are passed, the program uses time-based logic: it enables the filter between 10 PM and 6 AM.

using Microsoft.Win32;
using System.Diagnostics;

#if LINQPAD
var args = Environment.GetCommandLineArgs().Skip(1).ToArray();
#endif

var currentDateTime = DateTime.Now;

var atBrokerPath = Path.Combine(
	Environment.GetFolderPath(Environment.SpecialFolder.System),
	"atbroker.exe");

if (!File.Exists(atBrokerPath))
{
	Console.Error.WriteLine("This program requires the Assistive Technology Manager utility in Windows.");
	Environment.Exit(1);
	return;
}

using var colorFilteringSubKey = Registry.CurrentUser.CreateSubKey(@"Software\Microsoft\ColorFiltering");
if (colorFilteringSubKey != null)
{
	var active = 0;

	var firstArg = args.FirstOrDefault()?.Trim()?.ToLower() ?? string.Empty;
	if (firstArg.Equals("enable", StringComparison.OrdinalIgnoreCase))
		active = 1;
	else if (firstArg.Equals("disable", StringComparison.OrdinalIgnoreCase))
		active = 0;
	else
		active = (22 <= currentDateTime.Hour || currentDateTime.Hour < 6) ? 1 : 0;

	colorFilteringSubKey.SetValue("Active", active, RegistryValueKind.DWord);
	colorFilteringSubKey.SetValue("FilterType", 0, RegistryValueKind.DWord);
}

// https://learn.microsoft.com/en-us/answers/questions/862266/any-net-api-available-to-toggle-the-color-filter-w
var startInfo = new ProcessStartInfo(atBrokerPath)
{
	CreateNoWindow = true,
	UseShellExecute = false,
	Verb = "runas",
};
startInfo.ArgumentList.Add("/colorfiltershortcut");
startInfo.ArgumentList.Add("/resettransferkeys");

using var process = Process.Start(startInfo);
if (process != null)
	process.WaitForExit();