Calculate your current alertness level and map your personal alertness curve throughout the day — based on sleep, time, and circadian biology.
Alertness at any moment is the result of two competing biological processes. Process S (sleep pressure / homeostatic drive) builds continuously during waking hours — the longer you've been awake, the sleepier you become. Process C (circadian drive) follows a roughly 24-hour wave, peaking in alertness in the late morning and again in early evening, with a trough in the early afternoon and a steep fall in the late evening.
The alertness dip at 1–3 PM is real and biological — not caused by lunch. It reflects the circadian trough in cortisol and core body temperature. This is why napping (and siesta cultures) align with this window — the body is biologically prepared for sleep. Knowing your personal alertness curve lets you schedule demanding cognitive work during peaks and routine tasks during troughs.
Sleep debt elevates Process S continuously — meaning higher background sleepiness at all times of day. A person with 10 hours of accumulated sleep debt experiences the 1–3 PM trough at a much lower alertness baseline, often producing involuntary microsleeps (1–5 second sleep episodes) that impair task performance significantly. The circadian peaks are also lower, meaning their "best" window is substantially diminished.