Deep sleep is the most refreshing sleep, as subjectively described by people after they wake up. Unfortunately, the older we get the less deep sleep we get.
Deep sleep is stage 3 sleep (or stages 3 and 4 under the classification that includes a stage 4). On an EEG the voltage difference patterns over time show up as delta waves. This is also called slow-wave sleep to distinguish it from the fast-wave sleep of Stage 2. Deep, or heavy sleep, is so-called because it is more difficult to awaken people in this stage than in light sleep, and if woken suddenly from this stage, people have sleep inertia. People in deep sleep are less apt to wake in response to external stimuli than those in light sleep Sleepers in deep sleep move their bodies less than in light sleep, although more than in REM sleep. The restlessness of some sleepers that results in tangled-up bedding occurs in light sleep.
Deep sleep is a time of accelerated tissue repair. In growing children, this is a time of physical growth. Human growth hormone is released in the first deep sleep episode of the night, and the period is associated with rejuvenation. Common childhood sleep disorders such as nocturnal enuresis, night terrors, and sleepwalking happen during this period.
At some level, we crave deep sleep more than other types. If you stay up all night and go about your normal activities the next day, you have some sleep debt. But if allowed to sleep the following night as much as you need to feel refreshed, you will probably not double the time of your normal sleep. Rather, the normal sleep time is appended with an additional one-third to one-half of normal sleep period. So someone who normally sleeps 7 hours per night, may, after a missed night’s sleep, go for 10 hours before feeling back to normal. The interesting thing is the distribution of time among the stages during this recovery sleep. Pretty much all the lost deep sleep is recovered. The amount of deep sleep during this second night is twice what it is in a normal night. Lengths of REM and light sleep are lower. It seems that the body chooses to conserve slow-wave sleep as much as possible, and it more willing to sacrifice other stages of sleep.
An interesting experiment showed this desire for deep sleep bubbling up to the surface of waking behavior, however subconscious. Scientists monitored the sleep architectures of people with insomnia over an extended period of time. The insomniacs were not given access to their EEG results. The insomniacs were also given access to sleeping pills (benzodiazepines) and told they could take them if they felt they needed to. After nights when the people had lesser deep sleep, the subjects were more likely to choose to self-administer the sleeping pills. Stage 3 length was the single most important factor in predicting whether pills were taken. This suggests people have a feeling about their need for deep sleep.
How can you get more deep sleep? Unfortunately deep sleep can be elusive, and there is no way to directly force your brain to spend more time in this stage. The best you can do is prepare the ground for it and hope you will be blessed.
One way to prepare the ground is to get a full night’s sleep every night. Sleep medications can help, but remember that all sleep medications modify your normal sleep architecture. None directly lead to more slow-wave sleep pe se. They tend to result in increased time in light sleep, but light sleep might lead to deep sleep. Exercise, particularly strenuous anaerobic exercise such as lifting weights, may provoke more slow-wave sleep, although the evidence on this is not firm. It is probable that some people react to exercise by getting more deep sleep and others do not. The fact that tissue repair is at its fastest in deep sleep and human growth hormone release peaks during this stage cannot be overlooked. Aerobic exercise helps some people increase the length of time in deep sleep. There is some evidence that raising the body temperature, such as by soaking in a hot tub, increases predilection for stage 3 sleep. The effect of the aerobic exercise may be due to that fact that it raises body temperature. There are problems with this theory, including the fact that people tend to sleep better in cooler temperatures, stage 3 does not start until more than an hour after sleep onset during which time the body cools.
Is some stage 3 sleep deeper than others? Maybe. Generally, deep sleep or heavy sleep is called that only in comparison to light sleep. There is some physiological evidence that the "depth" of the sleep varies while the sleeper is in Stage 3. Monitoring of the brain can show instantaneous depth of sleep within Stage 3,
However, it should be emphasized that this variation is measurable only within one person. It is true that some individuals appear to sleep deeper than others in the sense that they are less apt to be woken by external stimuli, but there is no way to tell if any given person’s sleep is deeper or shallower per se than others.
One of the unfortunate effects of age is that as we get older we spend less time in deep sleep. While healthy people in their 20s spend 20% of the night in Stage 3, a typical 40 or 50 year old spends only 10% in that stage. By age 70 or 80, it is down to less than 5%, and sometimes maybe 2%. Other factors notwithstanding, you could look at a person’s EEG chart for a night and get an estimate of how old they are.
Why is this decline unfortunate? Less deep sleep is not just a result of aging; it appears to at least partially cause some of the negative characteristics of old age. Weakness, decreased mental acuity, infrequent feeling of refreshment in the morning – these are all connected to less deep sleep.