The Circadian cycle is the cycle between sleeping and wakefulness. The sleep cycle we generally refer to is the Ultradian cycle. This is the cycle between slow wave and REM phases of sleep (NREM – REM). NREM sleep is light sleep which is followed by deep sleep associated with a dream state of sleep known as REM. A cycle lasts around 90 minutes, and most people usually go through 5 – 6 of these each night (some people can have just 4 and others as many as 7). The sleep calculator figures out the start and end of each cycle to state the best times for you to go asleep.