Sleep Cycle - Bio-alarm clock that wakes you up at optimal times based on your sleep patterns.
Hourglass - Hourglass is the most advanced simple countdown timer for Windows.
Sleep as Android - Common alarm clocks ignore your sleep cycles, they wake you on schedule no matter your are currently in your deep sleep. Waking up from deep sleep is unpleasant, it makes you tired and it may negatively affect your productivity during the whole day.
wnr - Better than pomodoro, this timer app balances work and rest.
Pillow for iOS - An iOS companion that helps you sleep better 💤
Free Countdown Timer - Free Countdown Timer is a free, full-featured and user-friendly countdown timer for Windows