“You’ll feel better after a good night’s sleep.” Whether uttered as caring suggestion from a loved one, or by the bartender trying to end an awkward conversation, chances are that, whatever you’re dealing with, you will feel better after a long sleep. This is because sleeping not only allows our bodies to enter a different phase of consciousness, it also enables different phases of immunological signaling, hormonal coordination, metabolic balancing, cognitive assimilation, and emotional processing. These sleep-coordinated changes make us more resilient physically and mentally, allowing us to heal faster, make better decisions, and feel better. 

So, what is happening while we sleep that makes it so important? In this article, we look at the two sleep states we experience throughout the night—each with very different functions that are critical to our health. But first, what is sleep anyway?

What is sleep?

Scientists are great at defining innocuous things in accurate yet creepy ways, like sleep: “A state of immobility with reduced responsiveness, distinguished from coma or anesthesia by its rapid reversibility.”¹ That definition alone may cause insomnia. Besides rapidly reversible unconsciousness, another key feature of sleep is that the body tries (albeit, not always successfully) to recover any sleep lost during a period of sleep deprivation. The phenomenon of “rebound sleep” suggests that sleep isn’t a passive time of low activity or alertness, but rather a biological account that demands restitution upon going into debt.¹

When you feel drowsy, this is due to the gradual accumulation of various sleep factors that activate ‘sleep’ neurons and inhibit ‘wake’ neurons in the brain.² Essentially, once these sleep factors reach a certain level, you feel the need to fall asleep. Sleep factors are timed to accumulate at night by our circadian master clock (also in the brain), which senses light from the eye and transmits that information to the rest of the body.

We sleep to conserve energy and perform restorative activities

The vital function of sleep is most evident in studies of sleep deprivation. For example, in experiments with rodents and flies, sleep deprivation causes death more quickly than starvation!² In humans, sleep deprivation is associated with significant cognitive and emotional challenges, while poor sleep and day-sleeping (as with night shift work) is associated with metabolic and immune disturbances.^2,3

We know that one of the major functions of sleep is to conserve energy, especially in our metabolically demanding brain. While the brain is only about 2% of our body weight (depending on your body weight, right?), it consumes 20% of the body’s total energy.² When we’re sleeping, the brain decreases its energy requirement by 44% and its oxygen requirement by 25%.² Even so, the brain isn’t dormant during sleep; in fact, virtually all areas of the sleeping brain are active at various times.¹

As described next, our sleeping brain performs necessary restorative activities as it shifts between two sleep states, REM and NREM sleep. You may already know something about these states, such as that REM is when you dream and your eyes move back-and-forth, and NREM is “deep sleep”. But what you may not know is that these sleep states execute critical activities that help develop and maintain the nervous system.¹

Two types of sleep: REM and NREM

Rapid Eye Movement Sleep or REM

REM (Rapid Eye Movement) sleep is named for the weird phenomenon where we rapidly move our eyes from side-to-side at various times during sleep. Another weird thing about REM sleep is that it only happens in land mammals, and aquatic mammals that spend time on land. For example, fur seals experience REM sleep only when they’re out of the water.¹ There’s a fun-fact for your next cocktail party!

Another interesting fact is that REM sleep gets progressively longer as sleep proceeds, such that the longest duration of time spent in REM occurs just before waking.¹ During REM, people experience a paralysis of the postural muscles (the muscles that keep us upright), and the temperature of the brain increases.¹ Respiration and heart rate vary, and muscles get very twitchy.¹ Finally, the activity of neurons that manufacture certain neurotransmitters diminishes, which may indicate that they are conserving the energy required to synthesize them.¹

REM sleep involves very high brain activity

If you looked at the brain activity of someone during REM sleep, you would think that the person was awake. This is because REM sleep involves very high brain activity.⁴ It’s pretty incredible to consider that the primary neurological difference between REM sleep and wakefulness is the loss of environmental awareness and muscle paralysis, and that this is controlled by a small bunch of neurons in the brainstem!¹

Time spent in REM decreases with age

All of this neuronal activity must be used for something, and babies’ brains may be able to tell us something about its purpose. Human newborns spend about 50% of their sleep time in REM, down from about 80% while in the uterus.⁴ The amount of time spent in REM continues to decrease with age, and by 10 years old, children spend about the same amount of their sleep time in REM as adults, approximately 20%.⁴ While in REM, babies experience hundreds to thousands of muscle twitches in muscles that are otherwise paralyzed.⁵ The fact that these muscles twitches are observed in a period of temporary paralysis suggests that they are a special type of movement that promotes the development and maintenance of our sensory and motor nervous systems.⁵

REM sleep is important for processing emotions linked to memories

Previous research has established that REM sleep is important for processing emotions linked to memories; however, whether REM sleep is important for retaining new memories remains somewhat controversial within the scientific community (Learn more about REM and emotional processing in this article of our sleep series.) While REM’s role in memory retention still hasn’t been completely proven in humans, a clever experiment was performed recently with rats that demonstrates this role for REM sleep in rodents.⁶ The researchers had the rats learn a new task, and then fall asleep. Once the rats reached REM sleep, the researchers used a laser to non-invasively eliminate neurons involved in forming new memories. The rats whose neurons were eliminated during REM sleep forgot what they had learned. The rats whose neurons were eliminated after REM sleep retained the new information. This experiment tells us two important things: 1) new information is integrated during REM sleep, and 2) never fall asleep at a neuroscientist party. 

Non-Rapid Eye Movement sleep or NREM

NREM sleep is…non-rapid eye movement sleep? Yep, super technical. During NREM, your eyes don’t move rapidly from side-to-side, but they do slowly roll around.² (So why not call it SREM, for Slowly Rolling Eye Movement sleep?) You first enter NREM as you fall asleep, which occurs in 3-4 phases throughout the night. Each phase gets progressively deeper and is associated with diminishing brain activity.⁴ During an uninterrupted sleep session, sleep is mostly spent in NREM and transitioning between phases, while the time spent in REM gradually increases until waking. In humans, the transition from NREM to REM is abrupt, which is surprising considering the transition is from a low brain activity to a very high activity.⁴

NREM is known as recovery sleep

NREM has been called “recovery sleep” because this is when many critical molecules are built. The cells in our body constantly experience molecular turnover—the process of replacing damaged or depleted molecules with new ones. These molecules, such as proteins and fatty acids, are required for normal cell functioning. Another type of molecule that is built during NREM is glycogen, the storage form of glucose.² Having a source of glucose during sleep is important for maintaining our metabolism during the hours when we’re not eating. In addition to building molecules, NREM sleep is also the time when the connections between our neurons are modulated. Through the process of synaptic plasticity, the strongest connections between neurons are preserved and the weakest connections are eliminated.²

Read more about sleep

Sleep states are about much more than rolling eyeballs and twitchy muscles. REM and NREM are distinct periods of brain activity required for maintenance of the nervous system and the integration of new information.

Continue reading our sleep series to learn about the critical healthcare that sleep provides through our endocrine system and immune system, the importance of sleep to our mental and emotional health and how our biological clocks intimately link sleep with nighttime, and what happens to our bodies when sleep becomes misaligned with our internal clocks.