On the surface, sleep seems like a relatively easy process. You go to bed when you’re tired, sleep through the night (if you’re lucky), and wake up in the morning. Although you’re probably not aware of it, your brain goes through a series of four sleep cycles at night that are anything but simple. In fact, the brain is sometimes more active while it’s sleeping than when it’s awake.
When you don’t rest well, it’s easy to feel it the next day. You become overly tired, irritable, and unable to concrete to your best abilities. When this happens, it’s likely because you didn’t experience as many full sleep cycles the night before as you should have. Here is everything you need to know about the sleep cycle stages and how they affect your health.
Research shows there are three basic types of consciousness: wakefulness, non-rapid eye movement (NREM) sleep, and rapid-eye movement (REM) sleep (1). REM sleep is the most restorative form of sleep and is characterized by intense brain activity. During REM sleep, your brain waves move in a fast and desynchronized manner similar to the way your brain waves move when you’re awake. Your breathing increases and becomes shallow and irregular. Your eyes began to move rapidly in different directions, and your muscles become paralyzed. Your heart rate and blood pressure also increase.
REM sleep is the stage of sleep in which you experience dreams. It is also necessary for memory consolidation, mood regulation, and the synthesis and organization of thoughts (2). If you were to prevent someone from REM sleep but allow them to enter NREM sleep, they would continuously try to enter REM sleep and would spend an increased time at this stage.
NREM sleep includes the first few stages of sleep that occur before a person enters REM. It is characterized by a decreased state of physiological activity as a person falls deeper into their sleep cycle. During NREM sleep, a person’s brain waves, blood pressure and heart rate slow down until they enter REM sleep.
During slow wave sleep, it is possible for a person to experience sleepwalking, night terrors or bedwetting. Some research suggests that slow wave sleep helps the body recover and improves certain types of learning (3). A person’s sleep needs are based on the total time they spend in slow wave sleep. In other words, if a person has been awake for a long time, they will spend a longer amount of time in slow wave sleep.
After the three stages of NREM sleep take place, a person moves into REM sleep, which is the fourth and final stage of sleep. It usually occurs 90 minutes after a person falls asleep. A full sleep cycle lasts approximately 90 to 110 minutes long. Most people experience five to six sleep cycles a night. Achieving a proper balance between each sleep stage is an important part of maintaining health.
The four sleep stages work together so you can get the best rest possible. No one sleep stage is more important than the other. Restorative sleep is needed for optimal cognitive function such as memory, mood, learning and the ability to concentrate. Research shows that when one of the sleep stages is off, it may reduce longevity and health (2). Although each sleep stage is categorized differently, they essentially produce the same results and are completely reliant on each other.
Deep sleep is different from dream sleep (REM sleep). Although more importance is often placed on dream sleep, both stages are both equally as important for health. Deep sleep occurs during the third stage of sleep or N3. It is sometimes referred to as delta sleep because of the presence of delta brain waves, which are described as having low frequency and high amplitude.
Deep sleep is sometimes broken up into the two stages N3 and N4 depending on how many brain waves are present. Stage four or N4 is sometimes used to describe a sleep stage that has more brain activity than N3 but is not quite yet dream sleep. Recent research shows that there is no real difference between these two stages, so they are combined into the N3 sleep stage (4).
It’s very hard to wake a person from deep sleep. Sleepwalking occurs when a person experiences arousal that causes their brain to activate its motor center and wake up just enough to perform basic movements without being aware of it. Although deep sleep is most commonly present in the first sleep stage, it can occur in both N1 and N2. As a person’s sleep cycle continues, they move out of deep sleep into REM sleep. Due to temporary muscle paralysis, sleep walking is generally not possible in REM sleep.
Deep sleep is needed to satisfy the need for sleep that builds up over the day. This may explain why short naps are not always effective at making you feel better because you do not always enter deep sleep. On the other hand, if you sleep too long, it may affect your sleep at night because it decreases your sleep drive.
As an important part of the sleep cycle, deep sleep has many health benefits. Research shows that being woken from deep sleep causes a person to experience “sleep drunkenness” in which you feel sluggish and become unfit to drive (4). Deep sleep is also needed to get the brain ready for learning the next day.
The deeper the level of sleep, the stronger, slower and more synchronized a person’s brain waves will become. Your sleep cycle can be thought of as a progression of many different brain waves and back again. For example, the brain moves through beta and gamma waves when it’s awake to alpha and theta waves during the first few stages of sleep, to delta and slow wave sleep. Then it starts all over again.
Deeper levels of sleep also produce higher levels of arousal. While it’s relatively easy to wake up during the first two stages of sleep, it’s harder to do so in stage three sleep. As sleep progresses, your muscle tone deepens until you enter REM sleep and become temporarily paralyzed despite having low arousal and high brain wave activity.
Each sleep stage is responsible for certain neurological and physiological functions. They are so important to one another that when one stage is missing or interrupted, their duties are not carried out by other sleep stages. Sleep inertia occurs when a person seemingly has a sufficient sleep but they wake up feeling tired or disoriented. You may experience sleep inertia when the alarm goes off first thing in the morning, and you feel confused for a few moments as you find your bearings.
Unfortunately, deep sleep is affected by factors such as stress, age, and substance abuse. Preventing the amount of deep sleep you get increases the risk of premature aging, sleep apnea and other health problems (4).
Your sleep cycle causes your mind and body to undergo several changes as it progresses. The following sleep cycle graph is designed to show you how your brain waves are different depending on your activity:
Your sleep cycle changes quite a bit with age. Infants spend approximately 50 percent of their time in REM sleep (5). This amount steadily declines as you get older. By two years old, a child’s REM sleep time decreases to 30 to 35 percent. Normal young adults spend approximately 20 to 25 percent of their sleep time in REM sleep beginning at around age ten (5). After that, your sleep needs don’t change significantly until age 65. By the time you are 70 years old, your slow wave deep sleep is nearly gone (5). This is why many older adults spend most of their sleep time in lighter stages of sleep and wake up more often. It also explains why they nap during the afternoon.
Your sleep-wake cycle is controlled by the circadian rhythm, which is stimulated by a part of the brain known as the suprachiasmatic nucleus (SCN). Its job is to sense incoming light and send signals to the brain that synchronize the circadian cycle by telling you to either fall asleep or wake up. Approximately 15 percent of people complain about suffering from one of the following sleep problems:
A dyssomnia is a term used to describe people who have trouble staying awake and staying asleep. The most common sleep disorders include obstructive sleep apnea, restless leg syndrome, insomnia, and idiopathic hypersomnia (5). Insomnia and hypersomnia are mainly linked to depression and anxiety.
Lack of sleep has serious consequences on your health. Research shows that insomnia and other sleep disorders increase your risk of obesity, diabetes and heart disease. It also decreases your lifespan. According to one study, approximately 90 percent of people who don’t sleep well also suffer from another medical condition (6). Here are some other areas of your health that are affected by sleep loss:
The best way to improve your health and get in as many sleep cycles at night as possible is to develop a sleep routine that allows you to relax before bedtime. Turn off all electronics two hours before bed and don’t take them in the bedroom with you. Exercising earlier in the day or at least three hours before you go to bed can also help. Caffeine has been shown to stay in the body for up to ten hours, so be sure you enjoy your last cup of Joe well before sleep.
Finally, enjoy a light meal and don’t overindulge in alcohol. It may help to keep a sleep journal of your systems and consider taking a natural herbal remedy to help you relax. Other relaxing activities include taking a hot bath, sipping warm herbal tea, or reading a book. Some research shows that having sex before bed can also help you drift off to sleep (7).