What Time Should You Go to Bed?
Calculate your optimal bedtime or wake time based on 90-minute sleep cycles. Wake up feeling refreshed instead of groggy.
Why Sleep Cycles Matter
Sleep occurs in 90-minute cycles, each progressing through light sleep, deep sleep, and REM sleep. Waking up in the middle of a cycle — especially during deep sleep — causes grogginess (sleep inertia) that can last 30+ minutes. Waking at the end of a complete cycle, during light sleep, leaves you feeling alert and refreshed.
How Many Cycles Do You Need?
Most adults need 5-6 complete cycles (7.5-9 hours) per night. 4 cycles (6 hours) is the minimum for cognitive function but is not sustainable long-term. Less than 4 cycles impairs memory consolidation, immune function, and emotional regulation. The CDC recommends 7+ hours for adults.
Tips for Better Sleep
Keep a consistent sleep/wake schedule (even weekends). Stop screens 30-60 minutes before bed (blue light suppresses melatonin). Keep your bedroom cool (65-68°F / 18-20°C is optimal). Avoid caffeine after 2 PM (caffeine has a 5-6 hour half-life). Limit alcohol — it fragments sleep cycles even though it helps you fall asleep faster.
A 2003 University of Pennsylvania study found that subjects sleeping 6 hours per night for two weeks showed cognitive impairment equivalent to staying awake for 48 hours straight — but they did not feel tired, demonstrating that humans are poor judges of their own sleep deprivation.
