❓ Why?
One sleep cycle lasts 90 minutes on average, which means that if the whole amount of sleep is divisible by 1.5 hours, waking up will be easier.
A healthy person needs 5-6 such cycles: 7.5 or 9 hours of sleep, but even if it's not possible 3 hours of sleep (2 cycles) feels more full then 4 hours.
