Even though sleep seems like a passive process, it’s not. Sleep is an active state that is as complex as wakefulness. Your brain doesn’t shut down during sleep; rather, your brain is involved in a wide variety of vital processes geared toward its own repair and rejuvenation, restoring stamina, strength, and function to prepare for the day ahead.
All of the happenings in your sleep life are built into something that scientists call your “sleep architecture.” That’s a big term for how your sleep is laid out and the process that your body must go through before it feels completely rested.
There are two main types of sleep that you need to know about. Non-REM Sleep (NREM), which includes light and deep sleep, accounts for about 75 percent of your sleeping time. REM (Rapid Eye Movement) sleep is where you may spend some of your time dreaming. This typically takes place during the last third of your night’s sleep.
While you’re snoozing, you usually go through 4-5 sleep cycles that begin in light NREM sleep and end in REM sleep with each cycle taking between 70 and 120 minutes to complete.
Let’s check out what your body is doing during these stages of sleep and why they are so important.
Light sleep is the transitional stage between wakefulness and sleep. In this stage, you shift in and out of consciousness and lose a sense of time and place. Sleep onset—the process of falling asleep—takes place during this stage.
You’ll spend half of your total sleep in this stage, as the brain activity slows down from waking levels. The body also relaxes physically, as heart rate and breathing decrease. Light sleep is a state of full but not deep sleep, and it is still easy to wake from this stage.
Deep sleep, also known as slow-wave sleep, typically accounts for 10-20 percent of your total sleep time, depending on your age. During this stage, brain waves slow considerably. Heart rate and breathing slow, blood pressure lowers, muscles relax, and it becomes difficult to wake up.
Deep sleep is a critical time for physical restoration. Repair occurs at the cellular level, restoring strength and function to tissue, muscle, and organs throughout the body. During these sleep stages, the body also turns its attention to restoring function to the immune system.
You reach REM (Rapid Eye Movement) sleep for the first time approximately 90 minutes into the night. Periods of REM sleep start as brief, but REM sleep gets progressively longer throughout the night. During REM sleep, the brain significantly increases its activity levels compared to the other sleep phases. While in REM, your eyes move rapidly in different directions (hence, the name), heart rate and blood pressure increases, and breathing becomes fast, irregular, and shallow. Most dreaming occurs during this phase, and if you wake with an awareness of having been dreaming, you likely awoke from REM sleep.
REM sleep is a critical phase of sleep for learning and memory, a time when the brain consolidates, processes, and stores information. This is the mental restoration stage of sleep.
As you go through all of the sleep stages, it’s typical for your body to enter brief times of wakening. It’s perfectly normal to awaken between sleep stages, and often while in light sleep, your body is more susceptible to short periods of wake.
All stages of sleep are important. It is the balance of time spent in each sleep stage that is critical to feeling fully rested and refreshed, and to having the mental and physical energy to meet the requirements of the waking day. By creating habits and routines that promote healthy sleep, you help your body maintain the integrity of your own individual sleep architecture, to the benefit of your long-term health.