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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.
What does it mean to “sleep soundly”? What does it mean to “sleep deep”?
We can unpack these phrases with the scientific discoveries about sleep physiology of the past several decades.
“Sound” means whole and possessing integrity. Sound sleep is a description given to a night’s sleep that is satisfying to the sleeper. This is subjective, of course, and probably in relation to the sleeper’s experience of recent sleep. A sound sleep for a 50-year old might be described as troubling by a 20-year old. And that’s a good word for the opposite: troubling. The opposite of sound sleep is troubled sleep.
If we were to hook up a polysomnography machine to a person getting sound sleep, we would see few nighttime awakenings, no significant breathing problems, and a decent amount of time spent in both slow-wave deep sleep and REM sleep. “Decent amount” is subjective, but we can ballpark it as 90 to 120 min per night of deep sleep and 90 to 130 min per night of REM sleep for adults. (Too much REM is associated with depression. There doesn’t seem to be any such thing as too much deep sleep.) Sound also somewhat tangentially refers to the depth of sleep and how susceptible the sleeper is to awakening by noise. This concept is probably close to what people mean by sleep deeply.
At Tuck, we like to divide the night’s sleep into light sleep, deep sleep, and REM sleep (we didn’t invent these classifications.) Deep sleep is slow-wave sleep (called so because of the patterns of the EEG printouts) or Stage 3 sleep (according to one classification scheme) or Stages 3 and 4 sleep (according to other classification schemes).
What is the difference between quiet wakefulness and sleep? One difference is the ability or tendency to react mindfully to external stimuli. What is the difference between sleep and coma? Sleep is reversible; coma is permanent. But coma is also a state in which the subject cannot be awakened by shaking or loud noises, the way a sleeper can.
The depth of sleep is generally taken to mean how likely the sleeper is to be awakened from an external stimulus. Children are difficult to awaken not because they are lazy but because they are sleeping deeply. Even within a single stage of sleep (as measured by EEG) the depth varies as measured by sensitivity to acoustic stimuli. It has also been shown that people who have more “sleep spindles” on the EEG readings during a normal night of sleep have more tolerance for noise.
Also a hypnogram — the output of a polysomnogram — is typically printed so Stages 3 and 4 (the deep sleep) are shown lower on the page than Stages 1 and 2. This is just convention, but it further promotes the idea that Stages 3 and 4 are deeper sleep than the earlier stages.
By this measure, some people do sleep deeper than others. Compare two people in stage 3 slow-wave sleep and one may be easier to awaken than the other. Does this mean the deeper sleeper is getting more of the slow-wave sleep benefits than the lighter sleeper? No. The characteristics and benefits of different stages of sleep are relative to each other, not compared among people. Although some people are lighter or deeper sleepers than others (as measured by susceptibility to awakening), that doesn’t mean they are getting more or less out of sleep.
On an EEG measure, alpha waves (8-13 Hz) are a signature characteristic of the waking state. They significantly decline when the person falls asleep, but they do not disappear. Scientists have found that the level of alpha waves correlates with the depth of sleep. When alpha level is stronger, the sleeper is more susceptible to awaking from external stimuli.
Now what produces alpha waves is not understood. The thalamus section of the brain influences activity in the cortex that give rise to alpha activity, it appears, leading to a hypothesis that thalamus relays external stimuli to “cortical processing centers where it is capable of interrupting sleep”.