Enter your target bedtime and get your exact 10-3-2-1-0 countdown — last caffeine, last meal, last work, last screen, and lights out.
The 10-3-2-1-0 rule is a pre-sleep countdown developed by Dr. Craig Canapari, a pediatric sleep specialist at Yale, and popularized across social media as a simple framework for evening habits that support sleep. Each number represents hours before bedtime when you should stop a specific behavior. The rule works because each behavior addresses a distinct physiological mechanism that impairs sleep onset and quality.
10 hours — Last caffeine: Caffeine's half-life is 5–7 hours in most adults (meaning half remains active after this time). To reduce active caffeine below a sleep-impairing threshold by bedtime, stopping 10 hours before is a conservative but scientifically sound guideline. Caffeine blocks adenosine receptors — adenosine is the sleep-pressure chemical that builds throughout the day. Blocking it delays sleep onset and reduces slow-wave sleep depth even when you feel like you can fall asleep normally.
3 hours — Last large meal: Digestion elevates core body temperature and metabolic activity — both of which oppose the temperature drop required for sleep onset. A large meal within 3 hours of bed also increases acid reflux risk (which disrupts sleep through micro-arousals even when not consciously noticed) and keeps insulin elevated, blunting growth hormone release during early slow-wave sleep.
2 hours — Last work: Work — particularly cognitively demanding, stressful, or creative work — activates the prefrontal cortex and elevates cortisol. Cortisol has a biological antagonism with melatonin: when cortisol is high, melatonin production is suppressed. Stopping work 2 hours before bed allows cortisol to begin declining and the prefrontal cortex to deactivate, enabling the mental quieting needed for sleep onset.
1 hour — Last screen: Blue-spectrum light from screens (400–480nm wavelength) suppresses melatonin production by activating retinal melanopsin receptors that signal "daytime" to the suprachiasmatic nucleus. The effect is dose-dependent: 1–2 hours of bright screen exposure can delay melatonin onset by 30–60 minutes. Night mode and blue-light filters help but don't eliminate the effect — dimming screen brightness matters more than color filtering.
0 — No snoozing: The "0" rule means no hitting snooze. Snooze alarms fragment the final sleep stage, creating sleep inertia. The 5–10 minutes of additional sleep from snoozing is predominantly light stage N1 — virtually no restorative value — but the arousal-and-return pattern floods the system with cortisol repeatedly, making you feel worse than waking on the first alarm.