Sleep is divided broadly into two categories: REM sleep and non-REM sleep, sometimes referred to as NREM. You spend most of your night's sleep—about 75 percent of your total sleep time— in NREM sleep. The remaining 25 percent of sleep time is spent in REM sleep. NREM sleep is further divided into three phases. Together with REM sleep, these three sleep phases comprise the four stages of sleep. Throughout the night, you move repeatedly among these four sleep stages, in patterns that create distinct sleep cycles. A full sleep cycle typically lasts between 90-100 minutes. Over the course of a full night's sleep, you will generally complete 4-5 sleep cycles.
A graph of a night's journey in sleep shows movement in and out of the light and deep stages of NREM sleep and REM sleep, through a series of sleep cycles. This graph—called a hypnogram—reveals alternating peaks and falls that together resemble the silhouette of a city skyline. For this reason, sleep scientists refer to the overall progression of nightly sleep as sleep architecture.
In a normal sleep cycle, you move from the lighter stages of sleep to deeper before moving then into REM sleep, followed by a return to lighter sleep before a new cycle begins. Over the course of a night, the composition of sleep cycles change. Early in the night you will spend very short periods of time in REM sleep and comparatively more time in the deeper stages of NREM sleep. As the night progresses, sleep cycles gradually shift, extending periods of REM sleep and diminishing periods of deep sleep.
At each stage of sleep, the body is engaged in processes geared toward its own repair and rejuvenation, restoring stamina, strength and function to prepare for the day ahead. The restorative processes of sleep are physical, including cell repair, hormone regulation, and protecting healthy immune system function. Sleep not only restores us physically, but also mentally, aiding in the processing of memory, emotion, and learning. 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.
Stage 1 is the transitional stage between wakefulness and sleep. In Stage 1 sleep, 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 Stage 1. It is easy to be awakened from this stage of sleep. If awakened from Stage 1 sleep, you might not even be aware you'd been sleeping.
Stage 2 is a phase of light sleep. Over the course of a night and several sleep cycles, you will spend approximately 50 percent of your time in Stage 2 sleep as you move in and out of REM and the deepest stage of NREM sleep. During Stage 2 sleep, brain activity slow from waking levels. The body also relaxes physically, as heart rate and breathing decrease. Stage 2 is a state of full sleep, but not deep sleep: it is still easy to wake from this stage. During both Stages 1 and 2, the body relaxes as is prepares to move into deeper phases of sleep.
Stage 3 is a phase of deep sleep, also known as slow-wave sleep. Typically it takes about 30 minutes to reach Stage 3 sleep for the first time after falling asleep. During this phase of sleep, brain waves slow considerably. Heart rate and breathing slow, blood pressure lowers, and muscles relax. Stage 3 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 Stage 3 sleep the body also turns its attention to restoring function to the immune system.
You reach REM 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 increases its activity levels significantly compared the other sleep phases. Most dreaming occurs during REM sleep. 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 processes and stores information.
The distribution of sleep across stages changes over the course of a lifetime, from childhood through old age. A wide range of environmental and lifestyle factors—from light and noise to exercise, stress, and diet—can influence your nightly sleep architecture. Even your sleep routine itself affects how sleep cycles unfold: getting too little or too much sleep, as well as the quality of your sleep, can alter sleep cycles. 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.
Note: S+ is not a medical device. If you are seeking information on how to treat a sleep disorder, you should talk to your healthcare provider.