Sleeping is restful, right? It seems like our brains and bodies completely shut down while we sleep at night so we can be restored for the next day. While that might sound good, it’s only partly true. Sleep is actually an incredibly dynamic and complex process – and we absolutely need it if we’re going to be our best selves. As we learn more and more about what patterns emerge in our brains and bodies while we sleep, it becomes clear that sleep is actually a fairly active state, especially for our brains.
Sleep Cycles: The 4 Stages of Sleep
- Stage 1: Transition
- Stage 2: Light Sleep
- Stage 3: Deep Sleep
- Stage 4: REM Sleep
The process of sleep is fascinating – it has such a huge effect on our physical, mental and emotional health. Before we fully understood sleep, scientists once believed that nothing much went on in our brains and bodies while we slept. In fact, the widely held theory was that our brains simply shut down when we went to bed, only to reboot again each morning, just like a computer.
However, as we learn more about sleep patterns, we understand the complexities of what we go through during a full night of restful sleep and how important a good night’s sleep is to our overall health and well-being. In fact, there’s some indication that at times, our sleeping brain may even be more active than when it’s awake!
REM vs NREM
Our sleep can be divided into two major patterns – rapid eye movement, or REM, and non-rapid eye movement. NREM sleep gets deeper and deeper throughout its stages. On the other hand, REM sleep is lighter and is the stage of sleep typically associated with dreaming. It’s also necessary for appropriate brain development, especially for our youngest sleepers. REM sleep is considered an active sleep stage.
In healthy sleep, we typically move through each of these stages in order before starting over again, with each full cycle lasting around 90 minutes, but can last up to 110 minutes. We can usually move through a sleep stage in about 15-20 minutes. Throughout the night, we also may wake slightly between each stage of sleep. The first sleep cycles we go into generally are made up of short REM stages and longer periods of deep sleep, but throughout the night that flips. We might go through four or five full sleep cycles during a good night’s sleep.
4 Stages of Sleep Cycles
As we sleep, we experience four different stages. Let’s take a closer look at what happens to our brains and bodies during each one of these stages.
Stage 1: Transition
The first NREM stage of sleep is our lightest stage. It’s a drowsy sleep that usually starts not long after we go to bed. During this stage, our muscle tone relaxes, and our brain wave activity slows down. Because this stage is so light, we can be easily awakened during it. Stage 1 sleep is often characterized by slow eye movement. If you’ve ever had the sudden sensation of falling out of bed while sleeping, it likely happened during NREM stage 1, when it’s easy for sleepers to experience muscle spasms or hypnic jerks.
We’re not awake during this stage, but we’re also not deeply asleep. It’s very easy for sleep to be disturbed during this stage, and many people research how to fall asleep to make this transition occur more easily. This stage serves as our “sleep introduction,” and it’s relatively brief, lasting usually no more than seven minutes. This is the stage we usually stay in when we take a “cat nap.” If you wake up while in this stage, you may feel as though you haven’t slept at all. And while it might not sound like a whole lot is going on, this stage is important because it allows our bodies to ease into stage 2, which is the first quantifiable stage of NREM sleep.
Stage 2: Light Sleep
This stage makes up about 40-60% of our sleeping time each night. During stage 2, our brains begin to produce dramatic upticks in brain wave frequency known as sleep spindles. There’s some evidence that these sleep spindles help refresh our brains and prepare us for learning. During this stage, our brains also start to show the high-voltage, slow-wave activity patterns that mark stage three of NREM sleep. When people take “power naps,” they ideally wake up after this sleep stage. This stage is considered light sleep. Our body temperature begins to drop, and our heart rate slows as our body prepares to move into its deepest stage of sleep.
Stage 3: Deep Sleep
Stage 3 is when we get our deepest sleep. Known as more of a restorative phase, stage 3 is typically shorter than stage 2, generally accounting for somewhere between 5-15% of total sleep time for adults. This is our most restorative sleep – there’s generally no eye movement at all and very little muscle activity. During this stage, our brain waves are called delta waves – they are extremely slow and have large amplitude.
This stage in the sleep cycle is the least likely to be affected by outside stimuli, so waking up someone who is in stage 3 of a cycle can be difficult. And if you do wake up directly from stage 3, you’ll probably feel pretty disoriented for a few minutes.
While we spend less time in deep sleep than the other stages, it’s absolutely imperative for our overall health and well-being. For anyone suffering from sleep deprivation, they’ll spend extensive time in stage 3 sleep once they do finally fall asleep. Parasomnias such as sleepwalking, bedwetting, night terrors and talking in our sleep usually happen during stage 3, as well.
A Time for Healing
We don’t yet have the full picture of everything that goes on during this deepest sleep, but what we do know indicates that it’s extremely important. For example, we know that during NREM stage 3, human growth hormone is released, which restores our bodies from the stresses of our day. This is when our immune systems restore themselves. Our brains refresh and prepare for new learning during this part of the sleep cycle. Scientists also think that this stage is when the body repairs and regrows tissues, along with building bone and muscle. During this deepest sleep, we also experience cell regeneration and our blood pressure increases. Plus, glucose metabolism in our brains increases, which improves both short-term and long-term memory.
It’s essential to create the right kind of sleep environment that allows us to enter this deep, restorative stage of sleep multiple times through a night’s sleep in order to optimize our health and well-being. In fact, studies show that significantly missing out on stage 3 sleep may increase your chances of health conditions ranging from Alzheimer’s disease and diabetes to heart disease and stroke. If you’re wondering how to sleep better, everything from a good bedtime routine to the right support from your pillow and mattress can help you spend enough time in this important sleep stage.
Stage 4: REM Sleep
REM sleep can occur anytime throughout the sleep cycle, but it usually begins about 90 minutes after we fall asleep. Most adults will experience five to six REM cycles through a full night’s sleep. The first REM period of the night is also fairly short. Following a REM stage, the process starts all over again with stages 1, 2 and 3 intermixed before coming back to REM again for longer periods of time. Our first period of REM sleep typically lasts about 10 minutes. Each additional REM cycle gets longer, and the final one of the night may last up to an hour. During REM sleep, both our breathing and our heart rate pick up speed.
REM is considered the most active sleep state – during a REM period, our breathing is rapid, irregular and shallow, our eyes move around rapidly and the muscles of our limbs are temporarily paralyzed. Scientists think this is a defense mechanism that keeps us from trying to “act out” our dreams during this phase. Weird, right? Interestingly enough, EEG tests show that our brain waves increase to levels that are similar to when we’re awake.
During REM stages, our heart rate increases, blood pressure rises, and our bodies lose some of the ability to regulate temperature. It all actually sounds a little scary, but it’s a perfectly normal part of the sleep cycle. This is the stage in which we typically have our most vivid dreams. When we remember dreams from the night before, it’s usually because we’ve awakened during a REM cycle. People who live with sleep apnea may struggle most during this stage – the lack of muscle tone we experience during REM sleep can make it tough for the muscles around our airways to keep them open.
Rapid eye movement remains something of a mystery – it may be related to visual images from dreams, but scientific opinions vary. If you really think about it, the entire field of sleep science is relatively new. REM sleep wasn’t even discovered until 1953. Until then, most scientists thought that brain activity essentially shut down while we slept.
Today, we know that our sleep continues to move through these cycles throughout the night, some of which are extremely active. But why? We’re not sure. Some sleep scientists believe the regularity of the pattern has to do with both physical and mental recuperation, as well as memory consolidation – but at this point, it’s just an interesting theory.
How Does Age Affect Sleep?
Age can have a tremendous effect on our sleep cycles. In the most general sense, the amount of sleep we need tends to decrease as we age. While newborns require as much as 20 hours of sleep per day, that number declines to about eight hours in adulthood and decreases as we become elderly. It’s often difficult for elderly sleepers to get eight hours of sleep in one go. In fact, more than 50 percent of both men and women over age 65 report issues sleeping.
As we age, our bodies spend more time in the light sleeping stage. Older people experience various health issues and take medications that can disrupt sleep cycles. Daytime naps are more common as we age, which can reduce the quality of night-time sleep. In general, older people sleep less, have more fragmented sleep, and spend less time in both deep sleep and REM sleep than their younger counterparts.
Age-related sleep issues can be traced to several different sources. First, sleep patterns may become more irregular without the structure provided by a regular work routine. Certain health conditions, such as congestive heart failure, arthritis, Alzheimer’s disease, heartburn, and menopause also have the power to dramatically reduce sleep quality – and quantity.
Some older sleepers may deal with psychological difficulties or psychiatric disorders that affect whether they can consistently get a great night’s sleep. In fact, depression is twice as common for the elderly compared to younger adults, which can dramatically affect sleep. Life changes, such as the death of a loved one, moving out of a family home, or physical limitations can also contribute to stress and sleep difficulties. The risk of sleep disorders like sleep apnea also increases in old age, especially for women. All of these factors work together to shift the way our bodies function throughout our sleep cycles as we age.
Age also has an effect on the length of each sleep cycle. For newborns, each sleep cycle lasts roughly 50 minutes. This stretches to approximately 90 minutes per sleep cycle in adulthood. The younger we are, the more N3, or deep, restorative, sleep cycles we have during the night. By the time we’re elderly, we experience very little N3 sleep. Some scientists call the late childhood period the “golden age” of sleep – after age 11 or 12, people begin to experience more sleep disturbances. In fact, nearly seven out of every 10 adults experience some kind of sleep quality issue, which is why it’s so important to develop good sleep habits.
Many sleep problems go undiagnosed and untreated in adult and elderly adults because people assume that sleep problems are just a natural part of aging. But that’s not always the case – many treatments and approaches exist that can help adults of all ages get a better night’s rest. You don’t have to accept poor sleep as you get older.
What Affects Sleep Cycles?
Many different factors – both internal and external – can affect our sleep cycles. The stages of sleep are a sophisticated and complex system that can be influenced in different ways throughout the courses of our lives. Age, tiredness, exercise, stress, and time of day relative to your internal circadian rhythm all affect our sleep. Additionally, other environmental conditions, including temperature and light, and various chemicals (including nicotine and alcohol) can have a powerful effect on how our bodies cycle through sleep.
Stress can profoundly affect our sleep cycles, as can many medical conditions, especially those that result in chronic pain. What we eat and drink, the medications we take, and our overall sleeping environment can also make a huge difference in how our bodies cycle through the various stages while we’re asleep.
The Power of Light
Light exposure is one of the most powerful influencers of our sleep cycles. Exposure to light can cause our internal clocks to either slow down or speed up, which directly influences our sleep. Sometimes, light has a direct influence, like when a brightly lit room makes it difficult for us to sleep. Other times, it’s more indirect – for example, by impacting the calibration of our circadian rhythm and influencing our choice of bedtime.
Our retinas have specialized light-sensitive cells that tell our brains whether it’s daytime or nighttime – they’re the boss of whether our bodies want to wake up or go to sleep. Back in the day, we relied on the sun to determine our sleep schedule – we woke up at sunrise and went to bed after sunset. But with the advent of the electric lightbulb, our sleep rhythms began to change. Artificial light can fool our bodies into thinking it’s still daytime, making it harder to power down and fall asleep. Exposure to light at night can make us creep toward later and later bedtimes – which can make it difficult to wake up in the morning. And too much light in our rooms can cause us to awaken frequently, which prevents us from moving into the deeper sleep cycles.
Modern Life Challenges
Other modern life challenges, like shift work, jet lag, and caring for children during the night can take a major toll on our sleep cycles. People who work a graveyard shift and sleep during the day are, after all, living the sleep cycle exactly backward. Up to half of all people who work shift work report dozing off during their work shift at some point – which can be dangerous, depending on the nature of the work.
For example, air traffic controllers, nurses, doctors, airline pilots, police officers, and other public safety officials can struggle with the sleep deficit often created by shift work. And those who travel from time zone to time zone can feel completely wrecked when it comes to figuring out when they should sleep. These people often suffer from insomnia during the times it makes the most sense for them to sleep – and they also can feel excessively sleepy during the time they’re supposed to be awake since it’s directly counter to their body’s circadian pattern.
Our sleep history—the quantity and quality of our sleep in recent days – also can have a dramatic effect on how we move through the sleep cycle. For example, a poor night’s sleep, following a sporadic sleep schedule, can force our bodies to reshuffle our sleep cycles.
Arthritis and other chronic, painful health conditions can have a debilitating effect on sleep cycles. Both discomfort and pain tend to limit how deep sleep cycles are, along with how many times a sleeper might awaken between sleep cycles. People who deal with depression, anxiety or other disorders may find it hard to fall asleep and stay asleep – they may experience more light and REM cycles and struggle with how to get more deep sleep. Even the normal stress that we experience on a day to day basis can trigger our bodies to go into a fight or flight response, making sleep difficult.
Ironically, many of the drugs used to treat physical, mental and emotional conditions that disrupt sleep cycles are also, themselves, disruptive. Beta blockers, alpha blockers, and antidepressants, for example, are all known to decrease REM and deep sleep cycles and even encourage insomnia.
Understanding Our Sleep Cycles
Our bodies must have a strong sleep foundation based on the restful, restorative sleep stages in order to maintain optimal health and performance. While research has come a long way since REM sleep was discovered in 1953, there is still much we can learn and understand about how our brains and bodies cycle through a good night’s sleep. As we understand more about the physiological, chemical and physical processes we need for optimal restful sleep, we gain more power over how to shape our sleep lives so that they work to our very best health and well-being. We’ll sleep to that!