Sleeptly

Sleep cycle calculator

Wake up refreshed — not in the middle of a cycle.

Sleep moves through 90-minute cycles. Waking at the end of one feels dramatically easier than being yanked out of deep sleep. Pick your target time and we’ll do the math.

Go to bed at…

Each option ends a full 90-minute sleep cycle. We add 15 minutes for falling asleep. The two highlighted options give most adults the best balance.

Bed at
2:15 AM
3 cycles · 4h 30m of sleep
Bed at
12:45 AM
4 cycles · 6h of sleep
Best
Bed at
11:15 PM
5 cycles · 7h 30m of sleep
Best
Bed at
9:45 PM
6 cycles · 9h of sleep
Bed at
8:15 PM
7 cycles · 10h 30m of sleep
Bed at
6:45 PM
8 cycles · 12h of sleep

How it works

A healthy adult moves through four distinct sleep stages roughly every 90 minutes: light sleep (N1, N2), deep slow-wave sleep (N3), and REM. Early in the night, N3 dominates — that’s when your brain consolidates declarative memory and your body repairs tissue. Later, REM stretches longer and handles emotional and procedural memory.

Waking during N3 produces strong sleep inertia: grogginess, slower reaction time, low mood. Waking at the end of a cycle — when you’re briefly closer to light sleep — feels much cleaner. That’s the only thing this calculator does: it lines up your wake time with the end of a cycle.

The science

The 90-minute cycle figure traces back to Kleitman and Aserinsky’s REM-sleep work in the 1950s and has been refined by polysomnography studies since.

Sleep-duration recommendations used here come from the CDC and the National Sleep Foundation’s consensus report (Hirshkowitz et al., 2015), and align with American Academy of Sleep Medicine (AASM) guidance.

Sources: CDC: How Much Sleep· AASM consensus (PubMed)· NSF: Stages of Sleep

Frequently asked

How long is one sleep cycle?
A full adult sleep cycle averages about 90 minutes, moving through light sleep, deep slow-wave sleep, and REM. Cycles shorten and REM lengthens as the night progresses.
Why does this calculator add 15 minutes?
Most adults take 10–20 minutes to fall asleep (sleep latency). We use 15 minutes as a midpoint so the suggested bedtime is when you should be in bed, not yet asleep.
Is 5 or 6 cycles really best?
The CDC recommends 7–9 hours of sleep for adults, which corresponds roughly to 5 (7.5h) or 6 (9h) full cycles. Waking at the end of a cycle reduces sleep inertia (grogginess).
What if I only get 3 or 4 cycles?
Short-sleep nights happen. Ending on a cycle still beats waking mid-cycle, but chronic sleep restriction below recommended hours impairs cognition and metabolic health.
Does this work for shift workers or new parents?
The cycle math still applies, but sleep timing relative to your circadian rhythm matters more for those situations. See our shift-work guide for tailored strategies.