When it comes to sleep and dreams a few things are clear. Firstly, people really like to study them. Secondly, some of the gross physiological features of sleep can be observed and measured. Thirdly, despite all of this research, there is plenty of contraversy in the field of sleep research. Scientifically it’s a fascinating topic involving an altered physical and mental state that is somewhat independent of the conscious mind. It also seems to be another one of those ubiquitous phenomena in the human experience that we have trouble really putting our fingers on.
Before we talk about the things we know about sleep, lets talk about how we know them. The gold standard for measuring sleep phenomena is polysomnography. This technique measures three things. It encorporates an electromyogram (EMG), which is a measure muscle tone; an electro-oculogram (EOG), which is a measure of eye movement; and an electroencephalogram which is a measure of brain activity. In addition to these three measurements making up the polysomnograph, researchers often use direct observation to monitor gross muscle movements, or sensors that measure chest wall movements, leg movements, or oxygen saturation of lung and other tissues. Body temperature is also sometimes measured.
Like I said earlier, there are several basic things that we know about sleep. For example, there are two main kinds of sleep: rapid eye movement or REM sleep and non-REM sleep. Non-REM (N-REM sleep) can be further broken down into four stages and these are all defined by measurements of the polysomnophraph. First, if you’re drowsy but not asleep you have low voltage alpha waves on your EEG reading. Stage 1 N-REM sleep is characterized by low voltage theta waves and slow, asynchronous eye movements. This is the only stage of N-REM sleep in which your eyes are observed to move, and it is also the only stage of sleep during which you may not perceive yourself as having been sleeping upon waking. Stage 2 N-REM sleep is characterized by “sleep spindle” patterns and “k-complexes” in the EEG. “Sleep spindles”
are 1.5 sec long 12-14 Hz EEG waves that are generated when groups of nerves in your thalamus become synchronized by a pacemaker mechanism. I find this really cool because it reminds me of synchronization of your heart nerves and muscles by a similar internal pacemaker. It creates a regular rhythm by having the nerves work together. Stage three and four N-REM sleep show very slow delta waves. This is called deep sleep or slow wave sleep and stage four is basically just deeper than stage three.
In all of these stages of N-REM sleep the EEG, measuring brain activity, shows very different patterns than in wakefulness. These patterns tend to be both synchronized and rhythmic. In REM sleep, however, we see low voltage, high frequency waves that are similar to relaxed wakefulness. REM sleep is defined by the presence of this activated and desynchronized EEG pattern, rapid eye movements, and very low muscle tone. REM sleep also has two phases: tonic which is continuous and has the typical EEG pattern and muscle atonia; and phasic which is intermittent and involves the bursts of eye movement and irregular breathing and heart rate. About 80% of all dreaming happens during REM sleep although the two states are more loosely associated than previously thought.
As you fall asleep you pass through the stages of N-REM sleep in order and then into REM sleep after about 90 minutes. These cycles then repeat 4-7 times throughout the night with stage 3 and 4 N-REM sleep making up the largest proportion of normal sleep time and the transitional stage 1 N-REM making up the smallest. You spend about 20-25% of the night in REM sleep, although this number is larger for small children. Each stage involves less muscle tension than the one before and in all stages but phasic REM sleep the parasympathetic autonomic nervous system dominates.
The biological function of sleep itself is still under debate. We know that during N-REM sleep the body temperature is controlled at a lower set-point whereas in REM sleep temperature regulation ceases altogether. These effects have prompted the theory that sleep is important for the mechanisms of body temperature control. Another theory is that it is crucial for consolidation and maintenance of memory. This has been one of the most prevalent theories for a long time, however there is much contradictory evidence. Another theory is that it is necessary for general rejuvenation and neural growth. This theory is supported by the fact that REM sleep is crucial for CNS development in young animals, and the rather obvious observation that when you wake up you feel better than when you’re tired. Similarly, there are parts of the brain that actually undergo growth of new neurons throughout life, and sleep deprivation (specifically REM sleep deprivation) slows this neural growth.
If our understanding of the reasons behind sleep is sketchy, it’s nothing to the sketchiness surrounding our basic understanding of dreams. As I mentioned, dreaming has been closely associated with REM sleep, and this sleep stage is also poorly understood in terms of its biological role. While the other stages of sleep, as I was saying, can be explained as having a regenerative role on the body and mind, there is no obvious adaptive role for REM sleep. That is it’s hard to see what selective pressures would have resulted in this sleep stage developing through evolution.
People (at least adult people) can basically do without REM sleep. There are several pharmaceuticals in wide use that suppress REM sleep and people on these drugs undergo a massive reduction of REM sleep, basically to the point of eliminating this phase. These people do not suffer mental collapse, however, and seem to undergo no ill effects. In fact, some studies suggest that they show improvements in memory. Similarly, some people lose the ability to have REM sleep because of a brain injury and these people seem just fine too.
REM sleep is not just a human trait, however. All land animals and possibly all birds have REM sleep cycles, although of course we can’t ask the animals if they are experiencing what we consider dreams. Dreaming isn’t actually a necessary component of REM sleep in humans. Injury to a relatively small cortical region in humans eliminates the ability to dream while retaining REM sleep cycles. Some people also report never dreaming, although they also experience REM sleep. Small children experience a lot more REM sleep than adults, but they also report fewer dreaming experiences.
At one time, neurologists tried to explain dreaming as spontaneous and random neural signals originating from the part of the brain that generates REM sleep; This theory posited that dreams were basically a byproduct of REM sleep generation. A systematic investigation of dreams, however, showed that although they can be strange, they are definitely ordered and non-random and incorporate many components of waking life. This seems to suggest that they may have a role of their own rather than simply being a product of a different process.
To be honest, when I started researching dream science I expected to find a bit more scientific consensus on the subject. It does seem, however that the jury is basically out on everything from the specific role of sleep in general, to the reason we have REM sleep, to how closely connected REM sleep is to dreaming, and why we dream. I guess it’s not that surprising that dreams are hard to research when you consider them as an experiential rather than empirically measure-able phenomenon. The human mind, and the underlying neurological circuitry, is really a beautiful and complex thing. It’s a bit ironic that one of the hardest things to wrap our minds around is, in fact, our minds.