what are rem and nrem sleep

To Sleep, Perchance to Dream

 

Shakespeare likened sleep to death, the only difference being the types of dreams each might engender. Had the bard been knowledgeable about sleep cycles, Hamlet would have been more accurate in saying, “To sleep, perchance to dream, then not, then dream, then not, then dream…”

Sleep disorders, disruptions in the sleep cycle, are important to recognize and treat, because sleep is a very busy time of our lives.

 

The Sleep Cycle

There are basically two types of sleep alternating within a night’s sleep cycle. Rapid Eye Movement (REM) sleep and Non-rapid Eye Movement Sleep (NREM). The different stages have been classified based on the types of brain wave patterns evident in each. Below is a diary of this cycle from retiring for the night until awakening for the next day:

NREM Sleep

 

  1. N1: the transition from wakefulness to sleep. Eye movements are slow and rolling. It is the lightest stage of sleep. This stage should be no more than 5-10% of total sleep time, but with sleep apnea, patients can be stuck in this stage.
  2. N2: the largest percentage of sleep time, 45-55%. Sleep “spindles” occur, which are very short bursts of excited EEG waves. “K-complexes,” sharp dips in frontal lobe EEG waves followed by sharp spikes, occur during N2, lasting only half a second.
  3. N3: deep sleep, also called “slow wave sleep,” since it has low frequency/high amplitude delta EEG waves. N3 is usually 10-20% of total sleep, but this decreases with age. This is the most likely time for sleep walkers to get busy in whatever netherworld they are in.

REM Sleep

This is the time of vivid dreaming. It may be decreased with alcohol, sleeping pills, antidepressants, and other drugs. REM sleep makes up about a quarter of total sleep time.

 

  1. Phasic REM sleep: bursts of REM, increases in muscle tone (twitching) and in sympathetic activity.
  2. Tonic REM sleep: low muscle tone is consistent.

 

 

 

Leading a double life—what we do with the other third of our lives: memory consolidation and brainwashing

 

The more that is learned about sleep, the busier the brain turns out to be during this time. Recent research has indicated this is the time when the short-term memories of the day, stored in the hippocampus and amygdala, are converted into long-term memory. During this process, the frontal lobes—the “executive decision” part of the brain, decide which memories are important. For example, if after you step over an ant, sparing its little life but not watching where you’re going, you might bump into someone with very little tolerance for clumsiness who shoves you back so hard you fall and break your tail bone. Thirty years later, you won’t remember the ant, but surely will remember the miscreant who shoved you and made you sit down ever-so-gingerly for the next few years. You can thank your limbic system—the innermost sanctum of the brain—the hippocampus, amygdala, and other tissue types.

Here is what happened:

With your frontal lobes, you made the executive decision to step over the ant, weighing in an instant all of the pros and cons of sparing it, along with instantaneous ruminations on the meaning of life; this act, in real time, is imprinted in your hippocampus, where memories are constructed. Neurons that fire together to create the memory then wire together.Your amygdala creates an emotional memory of this, perhaps your delight in being omnipotent enough to grant the ant a reprieve from a crushing death via your footfall.

Unbeknownst to your frontal lobes that missed the memo altogether, you collided into the ruffian, constructing another memory in the hippocampus. The very next memory—immediately following, unfortunately—is that of the forceful shove and resultant fracture of your coccyx. Your amygdala now figures prominently in this cerebral sequence, making a very powerful emotional memory of anger, hate, and disgust for the horrible person who your frontal lobes have decided had clearly overreacted.

That night, you sleep. During REM sleep, your frontal lobes decide which memories in your hippocampus’s and amygdala’s info dump are worth keeping. Some neurons reinforce their synapses with others, while other neurons loosen up, depending on the memories that remain.

The ant is forgotten. Your tailbone pain is not. The shover’s face is not forgotten, either. Thirty years later you see him hobbling with a cane in pain. The inner reptile in us all—your limbic system—wants to shove him back. This guy’s own frontal lobes 30 years earlier had decided that, his own tailbone intact, shoving you was not something worth remembering, and it fizzled away as the synapses in his hippocampus failed to remain bonded. Rising above your primitive limbic system instincts, you make an executive decision to take the high road and, instead, tell him, “I’m glad you’re hobbling with a cane in pain. That’s good for ya!”

“Huh?” he responds. He does not remember you. You do not remember the ant. Or what you ate that day you were shoved. Or the million other little things that don’t matter.

 

The pause that refreshes

 

It is intuitive that sleep refreshes us. How? Even more perplexing, why? And why do we turn to crap when we become sleep-deprived?

New evidence supports the theory that sleep washes the brain of toxic metabolic byproducts. The brain is a busy place, eating up about a fourth of all the energy we manufacture for living. That’s a lot of sugar. And a lot of byproducts.

Most people, when thinking about the brain, they think of the gray matter—the neurons (nerve cells). But there is a whole ‘nother brain enmeshed among the neurons. These cells are called the “glia,” originally derived from immune cells. They surround the neurons and can act like a governor to excite or inhibit the synapses. At night, as we sleep through our phases, the glia are quietly taking out the trash. Spaces around them swell and there is an increase in exchange between the cerebrospinal fluid (CSF) and the gray matter, which results in the clearing of a lot of gunk built up from our previous thinking day. This also has been shown to clear beta-amyloid, the suspected culprit that builds up with Alzheimer’s disease.

 

How much sleep should people get?

How much sleep one gets depends on two things: duration and depth, that is, quantity and quality. The standard caveat is that any sleep that is insufficient in either duration or depth and which interferes with daytime alertness, performance, or health, is insufficient. This usually means 7-8 hours for adults and more the younger one is, so that preschoolers need 9-10 hours and infants need 11-13 hours a day.

Insufficient sleep is not the same thing as insomnia, unless the wakeup time is the same as compared to the varied fall-asleep time, which can decrease the quantity.

 

What are sleep disorders?

Sleep disorders are alterations in the sleep cycle that create a dysfunction in one’s waking hours. When alertness is compromised during the usual times of wakefulness, this can impact one’s quality of life tremendously, so it is important to identify any problems.

These, as well as the approaches to treat and remedy them, are discussed in another article discussing sleep medications and treatments.

 

In summary, it’s easy to appreciate that anything that disrupts the cycling of normal sleep will not only alter memory, but learning, too, which is important for school-age children. Additionally, a good night’s sleep is important in decluttering the brain of waste products. Sleep disorders can interfere with this, leaving us too sleepy during the day at best, and mentally dysfunctional at worst. The recent association between sleep disorders and general health is particularly disturbing, and underscores the importance of good sleep “hygiene.”

 

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