Lesson 1.11 · Unit 1 · CED topic 1.5 (part 1)
Consciousness, circadian rhythms, and the stages of sleep
You will spend about a third of your life asleep, and while you are asleep your brain is far from idle. Sleep research began with a surprise: in the 1950s, researchers watching sleeping people found that the brain cycles through distinct stages through the night, including a stage in which it is as active as when awake. This lesson covers what consciousness is, how the body clock sets the sleep and wake cycle, and the stages you pass through every night. The next lesson covers dreams, theories of why we sleep, and sleep disorders.
Learning objective.
- 1.5.A. Explain how the sleep/wake cycle affects behavior and mental processes throughout the day and night.
Essential knowledge.
- 1.5.A.1. Consciousness has varying levels of awareness of thoughts, feelings, behavior, and events in individuals’ internal and external worlds. Sleep and wakefulness are two types of consciousness.
- 1.5.A.2. The sleep/wake cycle is a circadian rhythm, which in humans is about a 24-hour cycle. Jet lag and shift work are disruptions of the circadian rhythm.
- 1.5.A.3. The stages of sleep are identified by their specific EEG patterns.
Consciousness
Consciousness is your awareness of your thoughts, feelings, behavior, and the events in the world inside you and around you. It comes in varying levels. When you are alert and focused on a task you are highly aware; when your mind wanders or you daydream, awareness is lower; and in sleep you are mostly unaware of the outside world, though not completely (a parent wakes at a baby's cry). Sleep and wakefulness are two types of consciousness. They are not simple opposites, because the sleeping brain is active in organized ways.
The sleep-wake cycle is a circadian rhythm
A circadian rhythm is a biological cycle that repeats about every 24 hours ("circa" means about and "dian" means day). In humans, the sleep and wake cycle is a circadian rhythm, and body temperature, hormone release, and alertness follow similar daily cycles. The internal clock is set by light. Light that enters the eyes tells the brain it is daytime, and darkness triggers the release of the hormone melatonin, which makes you sleepy. This is one reason bright screens late at night can delay sleep.
When the internal clock is out of step with the outside world, the result is a disruption of the circadian rhythm. Jet lag occurs after traveling quickly across time zones: your body clock is still on home time, so you are sleepy at the wrong hour and awake in the night. Shift work, such as working a night shift, requires staying awake when the body is programmed to sleep, and sleeping when it is programmed to be awake. Both reduce alertness and can harm health and performance.
In 1962 the French geologist Michel Siffre lived alone for two months in a cave deep in a glacier, with no clocks, no sunlight, and no way of knowing the time. He slept and woke when he felt like it and phoned his support team when he woke and when he went to sleep. When he came out, he believed less time had passed than really had (by several weeks), because his days had become longer. His sleep and wake cycle had settled into one a little longer than 24 hours, around 25 hours.
Reading the research: this is a case study of one person (and Siffre later repeated it with others). It suggested that the human body has an internal clock that runs even without cues from the sun, and that it is close to, but not exactly, 24 hours. Because it involved a single participant, the finding was later confirmed in studies with larger samples of volunteers living without time cues.
Measuring sleep: the EEG
Sleep stages are identified by their patterns of brain waves, recorded by an EEG (electroencephalogram), which records the electrical activity of the brain from electrodes on the scalp. Brain waves differ in speed and size. An awake and alert person shows fast, small waves (beta waves). A relaxed, drowsy person shows slower, larger waves (alpha waves). As sleep deepens, the waves become slower and larger still.
The stages of sleep
During a night, a sleeper goes through cycles of about 90 minutes, five or so cycles in an eight-hour night. Each cycle includes four stages of two types.
NREM sleep (non-rapid-eye-movement sleep) has three stages.
- Stage 1 is the lightest. The sleeper drifts off, breathing slows, and the waves are slow and irregular. People may experience hypnagogic sensations: vivid sensory experiences like a feeling of falling, a sudden jerk of the body, or seeing images as one is falling asleep. Someone woken from stage 1 often says they were not asleep.
- Stage 2 is a deeper light sleep, which takes up about half of the night. The EEG shows bursts of rapid waves called sleep spindles. The sleeper is harder to wake.
- Stage 3 is deep, slow-wave sleep, with large, very slow waves (delta waves). It is hardest to wake someone from this stage, and it is when the body does much of its physical restoration. Sleepwalking and night terrors are most common here.
NREM sleep is deepest early in the night, and it decreases in duration with each successive cycle.
REM sleep (rapid-eye-movement sleep) is a very different state. The eyes dart back and forth behind the closed lids, and the brain's EEG looks much like that of someone awake, with fast, irregular waves. Yet the body's large muscles are temporarily paralyzed, and heart rate and breathing are irregular. For these reasons REM sleep is called paradoxical sleep: the brain is highly active while the body is at its most relaxed. Dreaming typically occurs in REM sleep. REM sleep becomes longer with each cycle, so the longest periods of REM come in the final hours before waking. When a person is deprived of REM sleep (for example, by being awakened whenever they enter REM), the brain makes up for it later with extra REM, called REM rebound.
Eugene Aserinsky, a graduate student working with Nathaniel Kleitman at the University of Chicago, noticed that sleepers' eyes moved rapidly at certain times during the night. When he woke people during these periods, they usually reported vivid dreams. William Dement and Kleitman followed up by waking volunteers during REM and non-REM sleep and asking whether they had been dreaming. People woken from REM sleep reported dreaming in most cases (roughly 80 percent), and people woken from non-REM sleep did so rarely. Dement then woke people each time they entered REM, which led to REM rebound on the following nights.
Reading the research: the experiments compared reports collected under different conditions, using physiological measures (EEG and eye movements) rather than relying only on self-report, which strengthened the evidence. A limitation is that participants slept in a laboratory, which is not their usual setting, and dream reports are self-reports that depend on memory.
A graph of a night's sleep shows the sleeper in stage 3 for 40 minutes in the first cycle, 20 minutes in the second, and almost none in the last two. REM periods last about 10 minutes at the start of the night and about 40 minutes just before waking. What patterns do you see?
Step 1, deep (stage 3) sleep is longest early in the night and decreases across the night.
Step 2, REM sleep is shortest early and longest near morning, so it increases across the night.
Step 3, this matches what the exam expects: NREM duration decreases through the cycles and REM frequency and length increase as the night goes on.
Step 4, a practical implication: cutting a night short by a few hours takes away mostly the late REM sleep, which is why people who sleep too little may be missing much of their REM.
Identify the stage of sleep described by each. (a) The EEG shows large, slow delta waves and the sleeper is difficult to wake. (b) The sleeper is vividly dreaming and the EEG resembles wakefulness, but the muscles are paralyzed. (c) The person feels a sudden sensation of falling and a muscle jerk as they drift off. (d) The EEG shows bursts of activity called sleep spindles.
Step 1, (a) is stage 3 (slow-wave) NREM sleep.
Step 2, (b) is REM sleep, called paradoxical because the brain is active while the body is relaxed.
Step 3, (c) is hypnagogic sensation during stage 1.
Step 4, (d) is stage 2 sleep.
A student from California flies to London, eight time zones ahead, and has trouble sleeping at night and feels sleepy during the day for several days. Explain.
Step 1, the student's circadian rhythm is still set to California time. When it is bedtime in London (11 p.m.), it is 3 p.m. in California for the body clock, so the student is alert.
Step 2, in the morning in London the body clock is signaling that it is the middle of the night, so the student feels sleepy and groggy.
Step 3, over several days, exposure to the new light and dark pattern resets the clock, and melatonin release shifts, so the symptoms fade. This is a disruption of the circadian rhythm, like the effects of shift work.
- Consciousness has varying levels; sleep and wakefulness are two types.
- The sleep-wake cycle is a circadian rhythm (about 24 hours); jet lag and shift work disrupt it; melatonin helps regulate it.
- NREM stages 1 to 3 (stage 1 includes hypnagogic sensations; stage 3 has slow delta waves) and REM (paradoxical: active brain, relaxed body, dreaming). NREM decreases and REM increases across the night. REM rebound follows REM deprivation.
- Sleep stages are identified by EEG patterns.
- What is a circadian rhythm? Give an example in humans.
Show answer
A biological cycle that repeats about every 24 hours; the sleep and wake cycle is an example (as are daily changes in body temperature and hormone release).
- Why is REM sleep called paradoxical?
Show answer
The brain's EEG pattern looks similar to wakefulness and the brain is very active, but the body is almost completely relaxed (the large muscles are paralyzed).
- How do the proportions of NREM and REM sleep change over the course of a night?
Show answer
Deep NREM sleep is longest in the early cycles and decreases, while REM periods become longer and more frequent toward morning.
- A person is awakened every time they begin REM sleep for three nights, and on the fourth night is allowed to sleep. What would you predict about the fourth night, and what is it called?
Show answer
The person will spend more time in REM sleep than usual (REM rebound), as the brain makes up for the lost REM.
- A night-shift nurse struggles to stay alert at 4 a.m. Use the idea of circadian rhythm to explain.
Show answer
Her body clock is programmed for sleep at that hour (low body temperature and high melatonin), so working then conflicts with the circadian rhythm, reducing alertness.
- Why did Siffre's cave study suggest that the body has an internal clock?
Show answer
Without clocks or sunlight, his sleep and wake cycle continued on a regular schedule of about 25 hours, so the rhythm came from inside him rather than from outside cues.