Sleep basics, cycles and hypnograms By definition, sleep is “a naturally occurring, reversible, periodic and recurring state in which consciousness and muscular activity is temporarily suspended or diminished, and outside stimuli is reduced.”

 

Most people need a certain amount of sleep to function adequately, and the quantity varies substantially. The range usually falls within 4 to 9 hours with 7 or 8 hours of sleep per day being the norm. Quality of sleep is another consideration, as uninterrupted sleep is important in order for our brains to conduct its restorative functions. 

 

Research has found that most people undergo four of five sleep cycles when they sleep. Each cycle contains varying amounts of rapid eye movement (REM) and non-rapid eye movement stages. After falling asleep, the brain will enter the first cycle that is approximately 90 minutes in length, and it includes three stages of non-REM sleep, with the third stage being the deep sleep, or slow-wave stage.

 

After a sleep cycle, instead of waking up, the brain may enter a period of REM sleep, where dreams occur and the brain undergoes restorative functions. This cycle repeats another three or four times provided the sleep period is not interrupted. As the total sleep period unfolds, stage 3 sleep decreases, and REM sleep increases each cycle.

 

Hypnograms can measure the sleep cycles and the associated time that the brain is in each stage through brain wave monitoring. Knowledge of these sleep cycles provide the understanding that if a cycle is broken by being awakened, such as an emergency tone during the night, the sleep needed by the responder will not be as restorative as it could be. 

 

Numerous successive nights of enduring being awakened during sleep can lead to physical and mental exhaustion.