Sundial - Automatic Windows Light/Dark Mode at Sunrise & Sunset

Overview

Sundial is a lightweight Windows system-tray app that automatically switches Windows between Light and Dark mode at your local sunrise and sunset. It fills the gap left by Windows itself: Settings → Personalization → Colors lets you choose a color mode but offers no scheduling. Sundial adds that missing automation, running quietly in the background and keeping your desktop in sync with the sun—no window, no fuss.

The Problem

Windows users who care about their eyes and their aesthetic hit a frustrating limitation:

• Windows lets you pick Light or Dark mode, but there's no built-in schedule to switch automatically
• Manually toggling the theme twice a day through Settings is tedious and easy to forget
• Staring at a bright screen after dark strains the eyes; a dark screen in daylight can be hard to read
• Third-party solutions are often bloated, require admin rights, or demand manual time entry
• Fixed-time schedules don't track the changing sunrise/sunset across seasons

Users want their system theme to follow the natural day/night cycle automatically—accurately, quietly, and without hassle.

Solution

Sundial automates Windows theme switching based on real astronomical sunrise and sunset times for your location. It lives in the system tray, checks the time every minute, and flips the theme exactly when the sun does—light by day, dark by night—adapting automatically as those times shift through the year.

Core capabilities:

• Automatic switching — Light at sunrise, dark at sunset, every day
• Accurate sun times — Real sunrise/sunset from the astral library, anchored to your location's timezone
• Auto or fixed location — Detect your city from your IP, or enter exact coordinates
• System tray app — Runs quietly with a sun/moon icon; no window unless you open Settings
• Start on sign-in — One checkbox; uses the per-user registry key with no admin rights needed
• Instant shell refresh — Taskbar repaints immediately when the theme changes
• Optional app theme — Switch app theme alongside the system/taskbar theme
• Manual controls — Switch theme now or re-apply for the current time from the tray menu

How It Works

Behind the Scenes

1. Determine location — Auto mode resolves your approximate coordinates and timezone from your IP; Fixed mode uses coordinates you enter with your PC's timezone
2. Calculate sun times — The astral library computes today's sunrise and sunset, anchored to the location's own timezone for accuracy
3. Check every minute — Sundial compares the current time to sunrise/sunset and determines the correct mode
4. Write the theme — It sets SystemUsesLightTheme (and optionally AppsUseLightTheme) in the registry—the same keys the Settings app uses
5. Refresh the shell — A WM_SETTINGCHANGE broadcast makes the taskbar repaint immediately
6. Re-apply on launch — On startup, Sundial applies the correct theme right away so it's always accurate

User Experience

1. Launch — A sun/moon icon appears in the system tray
2. Configure once — Right-click → Settings to choose Auto or Fixed location and preferences
3. Forget about it — Sundial runs silently, switching themes as the sun rises and sets
4. Manual override — Use "Switch theme now" or "Re-apply for current time" whenever you like

Key Features

Automatic Theme Switching

• Switches Windows to Light mode at sunrise
• Switches Windows to Dark mode at sunset
• Checks every minute and re-applies on launch, so it's always correct
• Adapts automatically as sunrise/sunset shift through the seasons

Location Detection

• Auto (IP) — Detects your city and timezone from your IP address
• Fixed — Enter a latitude/longitude and use your PC's timezone
• Preview today's sunrise/sunset for the chosen location
• Fixed mode works fully offline

Convenience & Integration

• System tray only — No window clutter; runs quietly in the background
• Start on sign-in — One checkbox, per-user Run registry key, no admin rights
• Optional app theme — Toggle AppsUseLightTheme alongside the system theme
• Immediate repaint — Taskbar and shell refresh instantly on theme change
• Manual tray actions — Switch theme now, or re-apply for the current time

Settings Reference

Option	What It Does
Location → Auto (IP)	Looks up your approximate coordinates and timezone online
Location → Fixed	Uses the latitude/longitude you enter, with your PC's timezone
Start automatically	Launches Sundial when you sign in to Windows
Also switch app theme	Toggles AppsUseLightTheme as well as the system theme
Preview times	Shows today's sunrise/sunset for the chosen location
Switch theme now	Manual theme flip from the tray menu
Re-apply for current time	Forces the correct theme for the current moment

Use Cases

• Eye comfort — Automatically reduce glare at night and improve readability by day
• Developers — Keep the whole OS and apps in sync with day/night without manual toggling
• Writers & designers — A comfortable, context-appropriate workspace all day
• Remote workers — Seamless theme transitions during long work-from-home days
• Aesthetes — A desktop that always matches the time of day
• Laptop users who travel — Auto location keeps sun times correct wherever you are
• Offline setups — Fixed coordinates keep switching accurate with no internet

Technical Specifications

• Platform: Windows only
• Language: Python 3.9+
• Dependencies: pystray (tray icon), Pillow (icon rendering), astral (sun calculations), tzdata (timezones)
• Theme mechanism: Writes SystemUsesLightTheme / AppsUseLightTheme under HKCU\Software\Microsoft\Windows\CurrentVersion\Themes\Personalize
• Shell refresh: WM_SETTINGCHANGE broadcast for instant taskbar repaint
• Autostart: Per-user Run registry key (no admin rights)
• Config storage: %APPDATA%\Sundial\config.json
• Check interval: Every minute, plus re-apply on launch
• Distribution: Run directly via run.bat, or build a standalone Sundial.exe via build.bat (no Python needed on target)
• License: GPL-2.0+

Installation & Setup

Option A: Run Without Building

1. Install Python 3.9+ (tick "Add Python to PATH")
2. Double-click run.bat
3. A sun/moon icon appears in the tray
4. Right-click → Settings… to configure

Option B: Build a Standalone .exe

1. Double-click build.bat
2. Find the app at dist\Sundial.exe
3. Copy it anywhere and run it—no Python needed on the target machine

First-Time Configuration

1. Right-click the tray icon → Settings…
2. Choose Auto (IP) or Fixed location
3. Optionally enable "Start automatically" and "Also switch app theme"
4. Preview today's sunrise/sunset to confirm
5. Close Settings—Sundial runs quietly from here

How the Theme Switch Works

Sunrise/sunset → astral library (anchored to location timezone) ↓ Every minute → compare current time to sun times ↓ Registry write → SystemUsesLightTheme / AppsUseLightTheme (1 = light, 0 = dark) ↓ Broadcast → WM_SETTINGCHANGE → taskbar repaints instantly 

Why Choose Sundial

Feature	Sundial	Windows built-in	Fixed-time scripts	Heavy utilities
Auto light/dark scheduling	✓ Sunrise/sunset	✗ None	✓ Fixed only	Varies
Tracks seasonal sun times	✓ Yes	✗	✗ Manual updates	Some
No admin rights	✓ Per-user	N/A	Varies	Often needs admin
Lightweight (tray only)	✓ Minimal	N/A	✓	✗ Bloated
Works offline	✓ Fixed mode	N/A	✓	Varies
Standalone .exe option	✓ Buildable	N/A	✗	Some
Cost	Free	Free	Free	Often paid

Best Practices

• Use Auto if you travel — IP-based location keeps sun times correct wherever you are
• Use Fixed for privacy/offline — Enter coordinates once; no internet needed afterward
• Enable start on sign-in — So the theme is always correct from the moment you log in
• Enable app theme too — For a fully consistent light/dark experience across the OS
• Preview sun times — Confirm your location is set correctly after first configuring
• Build the .exe for other PCs — Distribute a single file to machines without Python

App Highlights

Set It and Forget It

• Configure once; runs silently forever
• Always correct thanks to per-minute checks and re-apply on launch
• Adapts automatically to seasonal changes

Lightweight & Respectful

• Tray-only, no window clutter
• No admin rights required
• Tiny footprint with minimal dependencies

Accurate & Flexible

• Real astronomical sun times via the astral library
• Auto or fixed location, online or offline
• Manual overrides always available

Repository Information

• Repository: github.com/towfique-elahe/sundial
• License: GPL-2.0+
• Platform: Windows
• Language: Python 3.9+
• Author: Towfique Elahe
• Author website: https://towfiqueelahe.com
• Status: Production-ready

What Users Say

• "Windows should have shipped this. My desktop now goes dark exactly when the sun sets—no more squinting at night." — Software Developer
• "Set it once and forgot it existed—which is exactly what I wanted. Runs silently in the tray and just works." — Remote Worker
• "Love that it uses real sunrise/sunset instead of a fixed clock. It follows the seasons automatically." — Photographer
• "No admin rights, no bloat, and I could build a single .exe for my other laptops. Perfect little utility." — IT Enthusiast

Getting Started

1. Install Python 3.9+ (or build the standalone .exe)
2. Run Sundial via run.bat
3. Right-click the tray icon → Settings…
4. Choose Auto or Fixed location
5. Enable start on sign-in and app theme switching if desired
6. Preview sun times to confirm
7. Let Sundial handle the rest

Roadmap & Future Development

• Custom offset (switch X minutes before/after sun times)
• Optional wallpaper switching alongside theme
• Multiple manual schedules in addition to sun-based
• Accent color switching
• Smooth transition notifications
• City search for fixed location (name instead of coordinates)
• Portable config export/import
• Signed installer distribution