A sleep cycle is a roughly 90-minute loop through three NREM stages and one REM stage. A typical night contains 4–6 cycles. Waking at the end of a cycle feels much more refreshing than waking mid-cycle.
Adults generally need 5–6 cycles (7.5–9 hours). 4 cycles (6 hours) is workable if you align with cycle-end timing. Anything below 4 cycles is likely to leave you tired.
Sleep studies show healthy adults typically take 10–20 minutes to fall asleep. This calculator uses 14 minutes as the midpoint, but you can adjust it to match your experience.