What is sleep?-The Science and Stages of Sleep

Sleep has long been considered just a “down time” when your brain and body shut down. However, after decades of intensive study, we have revealed the mystery of sleep: Sleep has distinct stages that cycle throughout the night in predictable patterns. The quality of your sleep and your health are not just related to your total sleep time but also to the sleep you get each night and the timing of your sleep stages.

Your brain and body stay active during the magical sleep process, and each sleep stage is closely linked to a unique pattern of electrical activity in the brain – brain waves.

Types of Sleep

There are two basic Sleep Types: Rapid Eye Movement (REM) sleep and non-REM sleep (with three different stages). Typically, sleep begins with non-REM sleep. During the first stage of Non-REM sleep, you experience light sleep and are easily awakened by noise or other disturbances. In this initial stage, your eyes move slowly, your muscles relax, and your heartbeat and breathing begin to slow. You then transit into the second stage of Non-REM sleep, which is characterized by slower brain waves with occasional bursts of rapid waves. You spend about half the night in this phase.

Your brain waves become slower as you progress into stage 3 of non-REM sleep, and your brain mainly produces slow, deep electrical waves called Delta waves. This stage is a deep sleep stage and is extremely difficult to be disturbed by external factors. Children who wet the bed or sleep walk tend to do so during stage 3 of non-REM sleep. Deep sleep is seen as restorative sleep and is essential for ensuring a feeling of rest and energy during the day.

Non-REM Sleep

Stage 1: Light sleep; easily awakened; muscles relax with occasional twitches; eye movements are slow.
Stage 2: Eye movements stop; slower brain waves, with occasional bursts of rapid brain waves.
Stage 3: Occurs soon after you fall asleep and mostly in the first half of the night. Deep sleep; difficult to awaken; large slow brain waves, heart and respiratory rates are slow and muscles are relaxed.

REM Sleep

Usually first occurs about 90 minutes after you fall asleep, and longer, deeper periods occur during the second half of the night; cycles along with the non-REM stages throughout the night.
Eyes move rapidly behind closed eyelids. Breathing, heart rate, and blood pressure are irregular.
Dreaming occurs.
Arm and leg muscles are temporarily paralyzed.

Brain and Body Activities During Sleep

During REM sleep, your eyes move rapidly in different directions although your eyelids are still closed. Your breathing also becomes more rapid, irregular, and shallow, and your heart rate and blood pressure increase. Dreaming typically occurs during this type of sleep, your arm and leg muscles are temporarily paralyzed so that you cannot “act out” any dreams that you may be having.

About an hour to an hour and a half after falling asleep, you typically first enter REM sleep, the sleep stages repeat themselves continuously during sleep. As you sleep, REM sleep time becomes longer, while non-REM sleep time in stage 3 becomes shorter. By the time you wake up, nearly all your sleep time has been spent in stages 1 and 2 of non-REM sleep and in REM sleep. If REM sleep is severely disrupted during one night, REM sleep time is typically longer than normal in subsequent nights until you catch up. Overall, almost one-half of your total sleep time is spent in stage 2 non-REM sleep and about one-fifth each in deep sleep (stage 3 of non-REM sleep) and REM sleep. In contrast, infants spend half or more of their total sleep time in REM sleep. Gradually, as they grow, the percentage of total sleep time they spend in REM continues to decrease, until it reaches the one-fifth level typical of later childhood and adulthood.

Why people dream and why REM sleep is so important are unclear. As we all know REM sleep stimulates the brain regions you use to learn and make memories. Animal studies suggest that dreams may reflect the brain’s sorting and selectively storing new information acquired during wake time. While this information is processed, the brain might revisit scenes from the day and mix them randomly. Dreams are generally recalled when we wake briefly or are awakened by an alarm clock or some other noise in the environment. Studies show, however, that other stages of sleep besides REM also are needed to form the pathways in the brain that enable us to learn and remember.

Better Sleep & Healthy Sleep Resource

Download the PDF version of “What is sleep?-The Science and Stages of Sleep (2)”, convenient for offline reading.