In this series, I dig a little deeper into the meaning of psychology-related terms. This week’s term is sleep. It’s something we all need to do, but what exactly is it?
Sleep involves altered consciousness and decreased reactivity to sensory stimulation from the environment. The brain’s energy consumption drops significantly, which allows for the repletion of adenosine triphosphate (ATP), a key form of fuel in the brain. The brain also produces more neurotransmitters.
The immune system, nervous system, muscles, and bones shift to an anabolic state, which involves building back up what might have been broken down (through the process of catabolism) during the day. Certain hormones, such as growth hormone and prolactin, are secreted during sleep.
REM & non-REM phases
There are two key phases of sleep: rapid eye movement (REM) and non-REM.
The non-REM stages, including the deep sleep phase, are a very quiet time in the brain. In the first (and lightest) non-REM stage, people will sometimes experience “hypnic jerks”, sudden muscle contractions preceded by a feeling of falling.
During slow-wave sleep, the deepest non-REM stage, it’s very difficult to rouse someone, and if awoken during that stage the person will feel groggy and there will be some level of impairment for about half an hour. The time spent in slow-wave over the course of the night decreases with age.
The REM phase, on the other hand, is a very active time. Besides the rapid eye movements that give this phase its name, the electrical pattern on an EEG (electroencephalogram) is very similar to what’s seen in someone who’s awake. However, the muscles throughout the body are mostly immobilized. Breathing becomes more irregular, and heart rate may increase. Dreaming occurs primarily during the REM phase, and more time is spent in REM during the later part of the night.
Long-term memory formation occurs while asleep, with the consolidation of declarative memory (information about things) occurring mostly during non-REM and procedural memory (how to do things) being handled mostly during the REM phase.
Dream duration can vary from seconds to half an hour, and on average people have 3-5 dreams per night with dream phases 60-90 minutes apart. While most dreams occur during REM sleep, some happen during deep sleep. Dreams during deep sleep are less vivid, and we don’t usually remember them. These are mostly associated with the hippocampus consolidating information into long-term memory.
During REM sleep, the brain does not release the neurotransmitters norepinephrine, serotonin, and histamine during this phase. The prefrontal cortex, which handles higher-level cognitive functioning, is less active while dreaming, which may be why dreams can feel very real even if they’re bizarre.
It’s unknown what part of the brain generates the vivid imagery of REM dreams. It is clear, though, that it’s not the same part of the brain that’s used for imagination while awake. Increased dopamine release is associated with increased vivid dreaming and nightmares, while suppression of dopamine transmission, such as with antipsychotic medications, is associated with decreased dreaming and less vividness. Antidepressants that affect serotonin can sometimes cause increased vividness of dreams. For people who have nightmares due to PTSD, there are some medications that could potentially help.
All mammals are thought to go through REM sleep, and signs of dreaming have been observed in a number of animals. I’m fairly sure that I’ve watched my guinea pigs while they were dreaming, with eyes darting around and ears flapping.
What do dreams mean?
The meaning of dreams from a neurophysiological perspective is unclear. Some researchers think that they’re involved in consolidating the day’s events into long-term memory. Others suggest that it’s the brain randomly putting together bits of information from stimulation of the posterior part of the brain.
Sigmund Freud believed that dreams were manifestations of one’s deepest thoughts and feelings, including repressed memories. Unsurprisingly, perhaps, he also thought that dreams were also a release of sexual tension. He believed that while in a dream state, the super-ego relaxed, and the id could really come out to play. Dream interpretation is an important element of psychoanalysis.
Dreams have been interpreted as having divine significance since early Egyptian times. Now, there are plenty of sites scattered around the internet that will tell you what each specific element of a dream means. There doesn’t appear to be any research to back up these various symbolic interpretations.
The body’s natural circadian rhythm has a significant effect on the timing of sleep-wake cycles. A circadian rhythm is one that:
- lasts approximately 24 hours and persists even in the absence of variations in light and dark
- can be reset through changing the environmental conditions one is exposed to
- involves body temperature adjustments
Even certain bacteria display circadian rhythms, so this is not something that’s unique to humans.
Light and dark play an important role in the circadian rhythm. There’s a direct connection between specialized photosensitive receptors in the retinas of the eyes and the suprachiasmatic nucleus (SCN) in the brain. The SCN acts as a regulator of the circadian rhythm, triggering the pineal gland to release melatonin when it’s nearing time for sleep.
I was curious about how this might affect my blind blogging friends, so I did a bit more digging. The specialized ganglion cells that pick up light in the retina are functionally distinct from the photoreceptor cells used for vision, so a small proportion of people who are totally blind and the majority of people who have some degree of light perception maintain a normal light-dependent circadian rhythm.
In mice, mutations in the “clock gene” can lead to overeating and obesity, with increased risk for diabetes. In humans, shift work and other activities that disrupt the circadian rhythm can contribute to weight gain, heart problems, and inflammatory conditions. Excessive screen time on electronics is another factor that can disrupt the circadian rhythm.
Sleep deprivation has both mental and physical effects. It increases the risk for type 2 diabetes, affects the release of the stress hormone cortisol, and has negative effects on memory and attention.
In a study involving lab rats, the rats died after 5 weeks of sleep deprivation. Sleep deprivation is sometimes used as an interrogation technique. The longest scientifically documented period a human has stayed awake without stimulants is 264 hours (11 days), while the Guinness World Record is 18 days, 17 hours.
Sleeping is essential for everyone, but for those of us living with mental illness, it’s even more important. It was only when depression first hit that I started having insomnia; before that, I was a great sleeper, which was certainly handy on long flights. Now my illness isn’t well enough controlled to be able to manage without sedating meds. Working can throw everything off, and sometimes driving home from work I’ll need to have the window open and music blaring to keep myself awake. The more days in a row I go without sleeping enough, the stupider I get.
Is sleep something you’re able to get enough of?
You can find the rest of the what is… series in the Psychology Corner.
- American Sleep Association: Sleep deprivation | Stages of sleep | What is sleep?
- How Stuff Works – Science: What are dreams?
- Wikipedia: Circadian rhythm | Dreams | Sleep | Sleep deprivation | Psychoanalytic dream interpretation
Sleep Better: The Little Book of Sleep is a mini-ebook that covers a range of strategies, both medical and non-medical, to help you get the best sleep you can. You can find it on the Resources page.