Sleep Calculator

The National Sleep Foundation recommends 7-9 hours for adults and 7-8 hours for those 65 and older (Hirshkowitz 2015). The lowest health risk in large studies sits near 7 hours. Very active people often need the upper end. Both too little and too much are associated with worse outcomes, so more is not always better.

A cycle moves through light, deep, and REM sleep. Its length is commonly quoted as ~90 minutes, but polysomnography studies put the average closer to 95-110 minutes, and it varies about 30% between people and across the night (Leger 2023). Waking near the end of a cycle, rather than mid deep-sleep, tends to feel less groggy, which is why these bedtimes are targets, not guarantees.

Your chronotype (early bird vs. night owl) mostly shifts the timing of your ideal sleep, not the total duration. Night owls forced to wake early tend to build up sleep debt. Aligning your schedule with your chronotype usually improves how rested you feel.

Regularly short sleep is associated with higher cardiovascular risk, weaker immunity, and impaired focus, and genetic studies support a causal short-sleep link. But the evidence is largely observational. Notably, each hour of long sleep above 7h carries slightly more associated risk than each hour below 7h (Yin 2017), and long sleep is often a marker of underlying illness rather than a cause, so treat a 9.5h night as occasional recovery, not a daily target.