The three types of sleep may look practically the same from outside, but they are actually surprisingly different.

Even though humans have slept since before the dawn of history, it is only a few decades since we found out that we pack three types of sleep into each night. (Well, a few Buddhist monks have managed to extend their consciousness into the realms of sleep and tried to describe them, but it is hard to understand for outsiders.) Today we know from electrodes on the head that we all go through these phases, every night for most of our lives.

The three types of sleep have very different functions. The deep, dreamless Delta sleep restores the body. In this sleep, a hormone is released that sets off a cascade of repair in the body, as well as growth in children. The slow brain waves also allows the brain to rest, in so far as it is possible.

The intense electro-chemical storm that fuels our most vivid dreams happens on the opposite end of the 90-minute sleep cycle. It is the REM sleep, after the rapid eye movements that are eerily similar to waking life. Also the heart and lungs are working hard to supply the bursts of activity that pass through our brain at this time, as the brain compares memories, combines them … and perhaps destroys them. One theory is that erroneous combinations are flushed from the system, which would explain why people who don’t get this type of sleep tend to go temporarily insane (hearing voices, becoming unable to control their emotions, feeling hunted and sometimes seeing things that no one else can see.)

Both the deep Delta sleep and the intense REM sleep are necessary for memories to settle properly in the brain. Unsurprisingly, perhaps, children have a lot of both, but as we grow up we have less and less of them. Some elderly people hardly have deep sleep at all, and very little REM. They may sleep less overall, but mostly it is the third type of sleep that expands to fill the night.

As adults, most of our night is filled with Theta sleep. The theta brainwaves are slower than alpha, which is the relaxed waves of contentment, meditation and the feeling of just being alive with no worries. Actually alpha waves also are found in sleep to some degree, especially shortly after falling asleep, but theta waves dominate. The experience of Theta sleep varies somewhat, but if woken you may remember drifting images without intense feelings and without much of a plot. Some people when woken from Theta sleep will say that they were not really asleep, they were thinking.

It seems likely that theta sleep has no “medical” purpose, but simply serves to conserve energy and keep us from waking up and becoming active at night. For our ancestors both of these functions were very important, of course. Food was often scarce, and humans have poor sense of hearing and smell compared to predators. Staying in the cave with a fire by the entrance was definitely the way to survive long enough to pass your genes on to the next generation.

In today’s brightly lit world, it would be nice if we could cut down on the theta sleep. Alas, it is hardcoded into our brain. Sleep happens in “cycles” of up to 90 minutes, although in some people they can be shorter. After we fall asleep, we spend a bit of time in Theta sleep, then slide down to Delta for the first time that night. After 15-30 minutes, brain waves become shorter and faster again and we spend most of our 90 minutes in Theta, before having a short intense REM sleep of perhaps 5 minutes. Then slowly down again for a second round of Delta, unless you are too old. This Delta is typically shorter than the first. The next time we come to the top of the sleep cycle, the REM episode lasts longer.

The third Delta is quite short in college students, which are the usual “lab rats” of research on humans. It is longer for teenagers and children, and entirely absent for most adults. But the REM periods get longer and longer for each sleep cycle, and can grow into long, elaborate adventures in dreams that leave us tired. Fueling a raging brain is hard work, and sometimes asthma attacks and heart infarcts happen in the last REM sleep of the morning, especially on weekends when people can sleep an extra 90 minutes cycle or two. Less dangerous but somewhat embarrassing, young men frequently have orgasm during their longest REM sessions. This is much less common in women – probably. A number of boys and men don’t actually remember the orgasm, but ejaculation of semen leaves a sticky proof for when they wake up.

With this understanding of the three phases of sleep, we have an idea why some people can get by on 3 or 4.5 hours of sleep – they need the delta sleep but for some reason require less REM sleep than most of us, or possibly they start having longer REM sleep already during their first sleep cycles. Generally, the more Delta and REM we manage to pack into the first sleep cycles, the less we need to sleep overall. This happens naturally to sleep-deprived people: Their first sleep cycles contain lots of delta, which is the first sleep debt to pay off, and REM also takes up more time than usual. Unfortunately, sleep deprivation is not a good solution, since it also has the side effect that sleep intrudes on waking life in short burst. These intrusions happen when we are sitting still in a static landscape: At school, at work, and on the highway.

Well, that seems like a good place to stop for this time. On the highway. We may have the technology to move faster than a speeding cheetah and with the weight of a rhino, but we still have the brain of a caveman. Especially when sleepy…