5 hours of sleep.

Five hours of sleep is the floor for short-term human performance. It\u2019s below the NSF-recommended minimum (7 hours) but above the point where most people begin to show gross impairment the next day. A single night at 5 hours will cost you about 10–15% of sustained-attention performance and a meaningful reduction in mood stability. Repeated nights at 5 hours will compound that cost rapidly. Five hours is roughly 3–4 sleep cycles. If your personal cycle length is 75 minutes, you get exactly 4 cycles in 5 hours (300 ÷ 75 = 4). If your cycle is 100 minutes, you get 3 cycles in 5 hours (with 0 minutes left over). This happens to be one of the cleanest 5-hour targets because you wake at a cycle boundary. Our calculator can match your cycle length to an exact 5-hour window that minimizes inertia. The practical guidance for a 5-hour night: choose a bedtime that gives you cycle-aligned wake time, avoid alcohol (which suppresses REM and makes the partial night worse), and commit to a full recovery night afterwards. If you\u2019re running a temporary 5-hour schedule for work (a conference week, a deadline sprint), limit it to no more than 3 consecutive nights. Five hours is also the shortest duration where a power nap can meaningfully bridge the gap. A 20-minute nap in the afternoon after a 5-hour night can recover most of the lost alertness — enough to perform a second full workday before crashing. Don\u2019t stack this trick more than twice in a row.