A very stimulating dream: This is sure to refer to REM sleep, which is important to regulate instincts. More about that below.

REM (rapid eye movements) is the name for the part of sleep where we have vivid and lifelike dreams; in fact, they may often feel “larger than life”, packed with emotional intensity.

It is thought that REM sleep fulfills two important functions. One is to integrate memories, the other is to balance instincts. Not only do people (and animals) learn more slowly without REM sleep, but they also become mentally unstable.

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For the casual onlooker, it may seem like sleep is one continuous state of inactivity. But science has shown that sleep consists of three main types which alternate through a 90-minute cycle. (It is not actually 90 minutes for everyone, and is generally longer toward the end of the night, so you cannot reliably use the 90-minute multiplier to calculate when you will be in which sleep phase. Sorry about that.)

There are considered to be five sleep phases, which fall in these three main types, and one of these again is the most peculiar of all. That is REM sleep. Both humans and many animals have this sleep type, and one can wonder how it got started in the first place. It certainly happened long ago, because the friendly egg-laying platypus spends more time in REM than any other mammal*, and they presumably parted ways with the marsupials and us while the dinosaurs were still having a good time.

(* REM sleep in monotremes, or egg-laying mammals, is actually mixed with slow-wave sleep. Perhaps our separation of them into distinct times is more efficient. We like to think so, but no one asked the platypus.)

With electrodes on the head, it is possible to find out when people are having REM sleep, and wake them up immediately. If one does this for a few nights, people will have trouble learning, and become steadily more irrational and experience mood swings. They will begin falling asleep briefly during daytime, and at that point go directly into REM sleep (similar to what happens during narcolepsy). Hallucinations can also happen.

Similar effects happen if you take too much drugs that suppress REM sleep. The most common of these is ordinary alcohol, but also several prescription drugs have this effect, including some that are used to treat insomnia! Talk this over with competent health personnel if you can afford to. Because of such side effects, sleeping pills should be used with extreme caution, and must not be combined with alcohol at the risk of insanity and death (presumably in that order).

Because REM sleep mostly happens toward the end of the night, our use of alarm clocks mean that we start cutting down on REM before the other types. The body will compensate for this, if we keep it up, by having more REM sleep earlier in the night than before. But this only happens when (and for as long as) we have a “debt” of REM sleep. It will make sure that the REM debt does not grow beyond control, but we still have some, and it costs us.

Because we integrate memories less well, we learn more slowly than we could have if we slept longer. This is especially true for long-term memory.

Because REM regulates mood, with less sleep we will be less happy and energetic than we could have been. The exception to this is a fairly common form of depression, in which REM sleep actually makes the patient more depressed. Have this checked with a doctor or two before you try to treat yourself.

Because REM acts as a regulator of instincts, we put on more weight if we lack this sleep. The alarm clock may be one of the contributors to the so-called “obesity epidemic”. It is easy to say that people should eat less and exercise more, but when you are hungry and tired, talk is cheap.

REM also helps regulate the sex drive. It is a well known observation that men usually spend their REM sleep in a state of visible sexual arousal, even if their dream does not have any overt sexual content. (This sometimes causes misunderstandings between spouses who sleep in the same bed.) With less REM sleep, sexual distractions gain more influence over men. (I don’t yet have information on how this works for women.) Unfortunately for those with sexual dysfunctions, the lack of REM sleep increases attention but not ability.

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REM starts as an “electric storm” near the brain stem and works its way through the primitive layers of the brain, which we share with reptiles and birds, then through the so-called “mammal brain” which we share with our furry friends. (In some animal species it stops partway through, but not in humans.) It is in these lower parts of the brain that REM regulates instincts and mood.

The intense activity then spread out through the neocortex, the large layer of brain that is specific to our species. This is where it causes vivid and strange dreams, which we may be able to assemble into stories that we call dreams. The process of making sense of dreams happens mostly after we wake up from them, and we only have a short time to do so before they fade. Because dreams integrate new memories with old – older the longer we sleep in the morning – we can use them to bring up memories not easily found in other ways.

If we don’t get enough sleep on workdays, it is common to sleep in on weekends and other days off (unless you have children, pets or other living alarm clocks.)  Some people are surprised that they wake up more tired when they have slept longer: Shouldn’t they feel more rested? No, not if you have REM debt. The brain will then pay off the REM debt by spending lots of times in this state once the alarm clock doesn’t go off. But REM is hard work for the brain. It is more intense than the thinking people are able to do at work, and second only to panic attacks in its sheer use of energy. To supply this brain workout, heart and lungs work harder than usual as well: Late in the morning sees a spike in asthma attacks and heart infarct. While the deep delta-wave sleep early in the night refreshes and rejuvenates us, and the “filler” theta-wave sleep lets us rest, REM sleep actually tires us.

I hope this little overview lets you appreciate the role of the mysterious REM sleep, and perhaps go to bed a little earlier or set the alarm a little later next time!