The sleep cycle is predictable, taking approximately 90-100 minutes, and consists of two distinct parts – non-rapid eye movement (NREM), with four stages of its own, and REM. Figure 2 shows an overview of what typical brain waves look like during each stage of sleep.
NREM Stage One: Very Light Sleep or “cat nap” - When you close your eyes after a tiring day, you’re in the first stage sleep, where your brain is dominated by alpha waves. In Stage 1, you can still be awakened pretty easily and this is sort of a “very light sleep” where you drift in and out of consciousness. Your muscle activity slows down and your eyes move slowly.
within minutes of falling asleep. This sleep is brief, lasting only a few minutes. You are somewhat alert and can be easily woken in this stage.
NREM Stage Two: Light Sleep or “power nap” - during this stage, the brain produces sudden increases in brain wave frequency known as sleep spindles followed by a slowing of brain waves. You are less alert, and it takes more effort to wake in this stage. Here, your eye movement stops, your brain waves slow down with an occasional burst of activity, and your body temperature drops. We spend almost half of our time in Stage 2!
NREM Stages Three & Four: Deep Sleep or “slow-wave sleep” (SWS) – during these stages, the brain begins producing slower delta waves. You will not experience any eye movement or muscle activity. At this point, it becomes more difficult for you to wake up, because your body is less responsive to external stimuli. The brain produces even more delta waves and you move into an even deeper, more restorative stage of sleep next. It is most difficult to wake up during this stage. This version of “restorative sleep” is when the body repairs muscles and tissues, stimulates growth and development, boosts immune function, builds up energy for the next day, and essential hormones are released.
REM Sleep: Most people enter REM sleep roughly 90 minutes after falling asleep with each REM stage lasting up to one hour. Even though we spend only about a quarter of our time in REM, that quarter is bustling with activity and “clean-up” functions. The average adult has five to six REM cycles each night. During this final phase of sleep, your brain is more active, but it prevents you from acting out your dreams as most of our dreams take place in this stage. The part of your brain responsible for sending nerve impulses throughout the spinal cord sends a message to turn off motor neurons, causing temporary paralysis so you don’t find yourself trying to recreate your day during the portion of the sleep cycle when most dreaming occurs. Your eyes jerk quickly in different directions which gives REM its name, heart rate and blood pressure increase, and breathing becomes fast, irregular, and shallow. REM sleep plays an important role in learning and memory function since this is when your brain consolidates and processes information from the day before so that it can be stored in your long-term memory. Even our breathing is irregular and shallow. During REM, energy is pumped to our brain and body to support daytime needs and we receive a boost to our immune system.
Figure 2. Brain waves during the stages of sleep.
While we sleep, a neurotransmitter called adenosine is lowered in concentration. Neurotransmitters are chemicals that relay signals between neurons and other cells and are responsible for focus, attention, energy, and more. Throughout the day, levels of adenosine increase as our brain functions, and this makes us sleepier. Our brains begin to lose battery. By the end of the day, they start to “power off”. This cycle allows our brain to block adenosine while we sleep, letting it “recharge” and prepare itself for the next day. In addition, alertness- inducing chemicals like adrenaline and dopamine are decreased in the evening while serotonin, melatonin (involved in the circadian rhythm) and gamma aminobutyric acid (GABA) are increased, allowing you to relax.
If we don’t sleep for a long period of time, what are the immediate and extended effects on our mind and body? What does science say?
While some conclusions are pretty hotly debated, let's examine the effects of sleep deprivation on our hormones. A lack of sleep definitely has at least some impact on insulin levels, androgens, growth hormone, and cortisol (the stress hormone).
Let’s look at insulin first (1), a hormone that everyone is at least a little familiar with. Through sleep deprivation, insulin sensitivity decreases in the fat cells. What this means is that the efficiency by which glucose is shuttled into cells goes down. When you get half of your normal sleep for less than a week, or even if you lose about an hour and a half for a few weeks, more insulin is needed to shuttle glucose into cells (2); less is shuttled into the liver and muscle cells and more is stored as fat. But do not fear! As long as you don’t stay up late every night, everything out of sync will be normalized in a matter of time.
Another hormone that can be greatly affected by losing sleep is testosterone (3). Studies vary in how much testosterone is affected, but getting three fewer hours of sleep for five consecutive days can reduce testosterone levels by over 10% to about 30%. As a result, you might feel as lethargic, suffer from poor concentration, and may not have the energy to live life as you see fit. Stages 3 and 4 of sleep coincided with more blood flow to muscles and a release of hormones. By depriving yourself of sleep, your ability to build muscle and burn fat as easily is obviously hindered.
In addition, growth hormone can be suppressed as well when losing sleep (4). Interestingly, a large pulse of growth hormone courses through your body after sleep begins, and in healthy young men, this accounts for 50% of daily secretion. While this may seem daunting, consider the fact that your body manages to compensate during daylight hours, and most growth hormonal imbalances can be replaced. Your body learns to adapt very well to inequities, though this should not be tested frequently.
Sleep debt can also impair muscle recovery and make injuries more likely. In one study, as a result of two types of partial sleep deprivation (5), there were some alterations of biomarkers related to muscle and cardiac injuries in response to acute intermittent exercise in athletes. What this means is that although no physiological response was noted, there were changes in the biology of the individuals that made injuries more likely to occur. While sleep deprivation does not affect cardiovascular or respiratory responses to exercises of varying intensity, you will become exhausted sooner (6). Protein is also stimulated more intensely to degrade, so reductions in testosterone will, in turn, decrease the activity of certain pathways that would normally promote protein synthesis and impair muscle growth (7). It may go without saying, but pushing your body too far constantly can give rise to injuries and illnesses.
If there’s one takeaway, it would be the effect of sleep deprivation on cortisol, the stress hormone. Cortisol is a hormone produced by the adrenal glands. It regulates energy by selecting the right type and amount of carbohydrate, fat, and protein needed by the body to meet physiological demands. Right after you work out, cortisol levels are higher than normal because fat or carbohydrate reservoirs provide energy to fuel your workout—these stores were mobilized by cortisol, depleting the body of the necessary nutrients. A few other functions of cortisol include aiding the maturation of “baby” fat cells into “grown” fat cells, suppressing the immune system, and increasing glucose concentration in the blood to make energy more available to muscles.
When you sleep less than normal, there’s a strain or stress that knocks on your body’s door telling it that something is not right. Cortisol levels rise to make more energy available to get you through your day, the stress goes down, and your body is once again in balance. But when you do not sleep, or sleep well, consistently for a period of time, that stress accumulates and cortisol levels are higher than ever, causing your body to think that you’re always hungry (8, 9). Specifically, cortisol binds to receptors in the hypothalamus of the brain, stimulating you to eat a diet that is rich in fat and sugar. An increase in appetite is only one part of the issues excess cortisol delivers, the other being fat storage.
There are more cortisol receptors in your belly, and this is apparent when you don’t get enough sleep as healthy peripheral fat is mobilized into unhealthy visceral fat in your abdomen. It is a vicious cycle. For those who are already overweight, cortisol’s fat accumulation effect is even more detrimental. This is why it is critical that stress is managed and prevented as much as possible with more sleep, deep breathing, yoga, meditation, other forms of exercise and healthy eating. If your cortisol levels are constantly elevated, fat stores can be relocated and deposited into the abdomen, contributing to and enhancing obesity and other problems. Figure 3 illustrates norms for one measure of estimating if someone is at a greater risk for visceral obesity, cardiovascular disease, type 2 diabetes mellitus or cerebrovascular disease: the waist-to-hip-ratio (WHR).
Figure 3. Standards for normal and abnormal WHRs.
The last point about hormonal effects deals with the relationship between sleep and metabolism. When comparing subjects who had four hours of sleep versus those who slept for ten hours, those with four hours of sleep were 23% hungrier, and were more likely to choose carbohydrate dense foods (10). Our brain gets tricked into thinking we are starved. However, it’s important to keep in mind that there are tons of conflicting studies, everyone’s genetic makeup is different, and there are a range of ways that we all respond to stress on our bodies.
If you toss and turn a lot, maybe these tips can help you get a good night’s rest (11). Avoid coffee, excess sugar and alcohol before bed time to get full sleep cycles and skip large meals before bed. Try not to use your phone or watch TV right before bedtime—clear your mind and keep calm before sleeping. Lastly, establish a regular sleeping pattern so your body will have a set clock.
Keep in mind that while the nature of some professions, like investment banking, sometimes demands a rigorous schedule bereft of sleep, it’s critical that we understand that getting adequate sleep does not make us “weaker” individuals. Sleep is a necessity and replenishes our body and mind. Otherwise, we are moodier, have less energy, and are at higher risk for chronic conditions like obesity, diabetes, and heart attack.