As you now know, sleep is an active rather than passive process. Let's go through the biological functions happening during sleep.
Very recently, it was discovered that we have the structure in our brains called the glymphatic system that functions mainly during sleep and is inactive or very poorly active during wakefulness.
The glymphatic system acts a waste management system for our central nervous system. It expands during sleep and contracts during wakefulness.
In an experiment using imaging in live mice, it was shown that sleep, whether natural or induced by anesthesia, is associated with a 60% increase in the size of the glymphatic system. This in turn was associated with an increase of interstitial fluid and increased rate of beta amyloid clearance during sleep. Beta amyloid is a protein that accumulates in the brain Alzheimer's disease.
What this all means is that sleep truly has a restorative function, because it helps clear the brain of accumulated proteins and potentially neurotoxic waste products that may contribute to unhealthy aging and neurocognitive disorders. One of the most important functions of sleep is its effect on learning and memory.
Memory is not a single entity, there are different types formed in different parts of the brain. It's usually divided into two categories. Declarative memory, which is conscious memory, like repeating a phone number or a name to help you remember it. This type of memory is encoded in the hippocampus. The second type is non-declarative memory, or implicit memory, which is what you use to learn how to ride a bicycle, for example. Declarative memory is included outside of the hippocampus in the cerebellum and basal ganglia.
It turns out that sleep plays a key role in memory formation. In particular, deep slow-wave sleep and REM sleep helps merge complex declarative memory. But does sleep have a role in non-declarative memory? Well, there is strong data to support that it plays a significant role, including areas of motor, visual, and auditory learning.
In some studies, human subjects were taught a hand motor task and then divided into groups, one learning the task in the morning and the other learning the task in the evening. What they found was that once sleep was introduced between learning and testing, subjects performed significantly better after sleep and retained much more than if trained in the morning and tested in the evening without a sleep period in between.
It also revealed that sleep deprivation actually interfered with learning, especially if stage two of sleep was lost, and that learning was much bigger with longer stage two phases. Similar experiments were done for visual and auditory learning. And again, subjects performed much better after a night of sleep, or sometimes even after a nap.
For visual and auditory learning, it seems that slow wave sleep and REM sleep are more important for learning and retention, and that disruption of these specific stages resulted in loss of the acquired knowledge.
The levels of hormones in the body are set by sleep, for example, cortisol, which is a stress hormone, surges in the early morning to wake you up and is suppressed by sleep. Another important hormone that is strongly affected by sleep is growth hormone. Growth hormone is responsible for growth in children and is important throughout life to repair tissue. Growth hormone secretion is maximized at night during deep slow-wave sleep.
Thyroid hormones also follow the sleep cycle. They peak after midnight and decrease during the day.