Humans don’t just sleep, but cycle through two completely different types of sleep.

The named these non–rapid eye movement, or NREM, sleep, and rapid eye movement, or REM, sleep.

REM sleep, is where brain activity is almost identical to that when we are awake, and it is intimately connected to the experience we call dreaming, and is often described as dream sleep.

NREM sleep is subdivided into four separate stages, unimaginatively named NREM stages 1 to 4.

Stages 3 and 4 are the deepest stages of NREM sleep we experience, with “depth” being defined as the increasing difficulty required to wake an individual out of NREM stages 3 and 4, compared with NREM stages 1 or 2.

NREM and REM—play out in a recurring, push-pull battle for brain domination across the night. The cerebral war between the two is won and lost every ninety minutes, ruled first by NREM sleep, followed by the comeback of REM sleep.

No sooner has the battle finished than it starts a new, replaying every ninety minutes.

(See attached graph taken from the book Why We Sleep).

While it is true that we flip-flop back and forth between NREM and REM sleep throughout the night every ninety minutes, the ratio of NREM sleep to REM sleep within each ninety-minute cycle changes dramatically across the night. In the first half of the night, the vast majority of our ninety-minute cycles are consumed by deep NREM sleep, and very little REM sleep. But as we transition through into the second half of the night, this seesaw balance shifts, with most of the time dominated by REM sleep, with little, if any, deep NREM sleep.

A danger resides in this sleep profile wherein NREM dominates early in the night, followed by an REM sleep dominance later in the morning, one of which most of the general public are unaware.

Let’s say that you go to bed this evening at midnight. But instead of waking up at eight a.m., getting a full eight hours of sleep, you must wake up at six a.m. because of an early-morning meeting.