It’s nighttime. You get into bed after a long day, and if everything goes as planned, you fall asleep and wake up in the morning refreshed and good to go; sounds simple, right? But the fact is, there’s a lot happening while we sleep, and like many other processes in life, our nightly rest is divided into different stages of sleep.
Sleep is divided into two main categories: REM sleep (Rapid Eye Movement) and non-REM sleep (sometimes referred to as NREM). During REM sleep, our eyes move quickly in different directions, and this is usually the part of the night in which we dream.
Usually, our night’s sleep is comprised of 75% NREM sleep and 25% REM sleep. NREM sleep is divided into three phases, and together with REM sleep these are the four stages of sleep. While the first two phases are referred to as the “light sleep” phases of sleep, the third phase and REM sleep are considered the “deep sleep” phases. Throughout the night, we move repeatedly between these stages, in predetermined patterns that create distinct sleep cycles. A full sleep cycle normally lasts between 90-100 minutes, and over the course of a full night's sleep, we generally complete 4-5 sleep cycles.
Between Wakefulness and Sleep
The first stage in this process is a transition between wakefulness and sleep, which normally lasts between 5-10 minutes. During this stage, we shift in and out of consciousness and lose a sense of time and place. It is sometimes referred to as ‘sleep onset’, the actual process of falling asleep. It is easy to be awakened during this time, and if awakened, you might not even be aware you had been sleeping. Additionally, the hypnic jerk we sometimes experience – the phenomenon of suddenly jumping and awaking due to involuntary muscle twitches, occurs during this stage.
Light Sleep Phase
Over the course of our sleep, we spend about half of the night in this phase as we move in and out of REM sleep and the deepest stage of NREM sleep. During this phase, brain activity slows down, heart rate and breathing also become slower, muscles relax and body temperature drops - preparing our body for deep sleep. Moreover, blood pressure and other metabolic functions decrease and slow down. The light sleep phase is a state of full sleep, but not deep sleep, which means you’re still likely to be awakened pretty easily during this time.
Deep Sleep Phase
The third phase of sleep is deep sleep, also known as ‘slow-wave’ sleep or ‘delta sleep’. It usually takes about 30-45 minutes to reach this phase after falling asleep. During this time, brain waves slow down as well as heart rate and respiration; blood pressure drops, the muscles fully relax and it is significantly harder to be awakened; however, if woken up, you are likely to feel disoriented for a few minutes. The deep sleep phase is critical for physical restoration. Repair occurs at the cellular level, restoring strength and function to tissues, muscles, and organs throughout the body. During this phase the body also focuses on restoring function to the immune system.
Rapid Eye Movement
After about 90 minutes of sleep, we reach the REM sleep phase. Periods of REM sleep gradually become longer throughout the night, with the longest periods occurring in the early morning hours (the last REM period may last up to an hour). During REM sleep, the brain significantly increases its activity levels compared to the other sleep phases. Heart rate and breathing also increases compared to other sleep phases.
The majority of dreaming occurs during REM sleep. If you wake up with an awareness of having been dreaming, it is most likely that you awoke during this phase. REM sleep is a critical phase of sleep for learning and memory consolidation, a time when the brain processes and stores information. Since this is also the last phase of sleep, waking up with an alarm clock could mean that you might be losing important REM sleep. Therefore, naturally waking up (i.e. at the end of a sleep cycle) is preferable to waking up with an alarm clock.
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