Circadian Rhythm — How Your Body Divides the Day
Know your peak hours and your day changes
Human alertness isn't constant throughout the day. The general pattern:
8 AM - 12 PM: Peak cortisol. Highest alertness and focus. Best for complex decisions and creative work.
1 - 3 PM: Circadian dip. Alertness naturally drops. This is hardwired into your biology regardless of nighttime sleep.
4 - 6 PM: Second alertness peak. Not as strong as morning but focus returns. Body temperature also peaks, making it good for exercise.
After 9 PM: Melatonin production begins. Alertness gradually declines.
This varies by person. The "morning lark" vs "night owl" split is largely genetic (chronotype). Rather than forcing yourself into morning mode, figure out your peak hours and schedule important work there.
Light is the master switch
The strongest signal for setting your circadian clock is light. Getting natural light within 30 minutes of waking resets your internal clock. Even on cloudy days, outdoor light (~10,000 lux) is 20x brighter than indoor fluorescent (~500 lux). 5-10 minutes is enough.
Action Points
Get natural light within 30 min of waking (5-10 min)
Schedule most important work during morning peak (8-12)
Use the afternoon dip (1-3 PM) for simple tasks or meetings
Avoid bright lights after 9 PM