THIS PAPER is intended to draw attention to a basic biological cycle—the 90-minute "sleep-dream cycle"—to define and describe the cycle, and to present some data from studies specifically investigating this variable.
The outline of a typical night's sleep defined by electroencephalogram and eyemovement criteria is by now well known. The parameters usually studied are the amount of sleep, the amount of D-time (rapid eye movement REM)-time, "dream time") in absolute terms or as a percentage of the night's sleep, the amount of stage 4 or other stages of "slow-wave sleep," the number of awakenings, and time spent awake. As seen in Fig 1, a D-period occurs approximately every 90 minutes during the night, and it is this cycle—90 minutes long in adult man—which will be our subject here.
Characteristics of the Cycle in Man
The cycle and investigations of changes in cycle length
Hartmann E. The 90-Minute Sleep-Dream Cycle. Arch Gen Psychiatry. 1968;18(3):280-286. doi:10.1001/archpsyc.1968.01740030024004