The 90 Minute Cycle
Let’s break it down. Sleep takes place in ninety minute cycles. Each ninety minute cycle is broken down into six phases. In the first phase known as latency or ‘pre-sleep,’ we are just beginning the process of going unconscious. During this phase, two neurotransmitters (adenosine and GABA) as well as a hormone called melatonin (more about this later) act to lower body temperature, slow down brain activity, and inhibit body processes associated with wakefulness. We are not yet asleep, but we are no longer fully awake. Latency is followed by Phase I sleep, the first level of unconsciousness. At this point we no longer have awareness of our surroundings–we are asleep. Each of the four successive phases represent progressively deeper levels of sleep.
As we go into the deepest levels the body undergoes profound physiological changes, releasing certain hormones and neurotransmitters, inhibiting the release of others, refreshing our brain circuitry, mobilizing the armed forces of the immune system to fight viruses and bacteria which infect us each day. During deep sleep our bodies go to work repairing stressed and damaged muscle, tendon, ligament, and skin tissue, and burning fat from our waists. Paradoxically, the deepest phases of sleep are also the most fragile; they can be easily disturbed by movement, noise, light, or other stimuli.
The sixth (deepest) phase of sleep, called ‘REM’, is the phase during which we dream. It is also the phase during which the most important physiological restoration takes place. We cycle down to REM sleep and then back up to phase one, light sleep, over a period of ninety minutes. As adults, in order to get full restoration of our health we must have a minimum of four of these ninety minute cycles each night, but ideally we should have five to six of them. Each successive cycle includes more time spent in REM.