Reading:  p. 190-214

 

 

We spend on average 1/3 of our lives asleep.

 

When we go to sleep, we pass through 6 characteristic stages that can be distinguished on the basis of the predominant brain waves (measured with an EEG).

 

We also experience characteristic changes in muscle tonus (measured with an electromyography, or EMG), and we experience rapid eye movements (REM) when we dream, which can be measured with an electro-oculogram (EOG)

 

6 Stages of Sleep

Stage 0—relaxed with your eyes closed, trying to fall asleep.  Characterized by alpha waves—regular, medium frequency waves of 8-12 Hz and beta waves—irregular low-amplitude waves of 13-30 Hz.

 

Stage 1—begin to drift off, characterized by theta waves (3.5-7.5 Hz).

 

Stage 2—begin to see sleep spindles and K-complexes.  Sleep spindles will occur in all stages, and are short bursts of waves 12-14 Hz.  Sleep spindles may be required to keep us asleep; older people have fewer sleep spindles, and generally awaken more during the night.  K-complexes are only seen during Stage 2, and are sudden, sharp waves that appear to be forerunners to delta waves (which begin in stage 3).

 

Stage 3—delta waves begin, high amplitude, low frequency waves (less than 3.5 Hz).

 

Stage 4—characterized by more than 50% delta waves.  Stage 3-4 are often referred to as “slow-wave sleep” due to the presence of slow delta waves.  Stages 1-4 are collectively called non-REM sleep to distinguish them from the last stage of sleep, REM.

 

REM—about 90 minutes after stage 1, enter this stage.  Characterized by theta wave activity, rapid eye movement, and a loss of muscle tonus to the point of being paralyzed.  Also see some beta wave activity, which is why REM sleep is often called paradoxical sleep—you’re in a deep sleep, but your brain waves look “awake”.

 

REM sleep is also accompanied by dreams—if you awaken during or just after a REM period, you can almost certainly report a dream, although often you won’t realize you were dreaming.

 

We dream every time we experience REM, and we go through REM 4-6 times per night.  Thus, we have 4-6 dreams every night (we just don’t remember them all).

 

 

Why do we sleep?

Slow-wave sleep appears to give the brain time to rest.

REM sleep appears to promote brain development and learning (i.e., LTP), although these mechanisms are not clearly understood

 

All species sleep, even when sleep should probably not have been selected for.  Some species of dolphin and porpoise sleep with one hemisphere at a time.

 

Humans deprived of sleep are not impaired physically, but are definitely impaired cognitively—problem-solving skills, concentration, and memory are disrupted, and most people will experience perceptual distortions or hallucinations after prolonged sleep deprivation (24+ hours).

 

While we can’t and don’t “make up” for lost hours of sleep, we do spend proportionally more time in slow-wave and REM sleep on nights following sleep deprivation.

 

Slow-Wave Sleep

Brain appears to “rest” during stage 4 sleep—brain activity is very slow (delta waves), as is blood flow and metabolic activity during slow-wave sleep.  Regions of highest activity when awake demonstrate regions of lowest activity during stage 4 sleep.

 

Can lack of sleep kill a person?  It can kill laboratory animals, and in the inherited condition of fatal familial insomnia, humans die.  However, it is not known if death is caused by lack of sleep in the condition, or if death is the result of other types of brain damage (the disorder damages portions of thalamus).

 

When sleep deprived, rats failed to groom normally, ate much more than normal but increased metabolic activity so they lost weight, and became unable to regulate body temperature.  Eventually the rats died, although it is unclear what caused death since their brains and levels of stress hormones were normal.

 

Vigorous exercise compared to no activity has no effect on duration or quality of slow-wave or REM sleep.

 

However, mental activity increases the duration of stage 4 sleep.

 

REM sleep

If we’re deprived of just REM sleep, experience the REM-rebound effect—spend a much greater than normal amount of time in REM sleep the next night.  Thus we must need REM for something, and there must be a regulatory mechanism determining how much we get.

 

Developmentally, infants/children experience much more REM than older or elderly adults—thus REM might have something to do with brain growth.  Evidence shows that species born with well-developed brains spend less time in REM than species with less developed brains

 

While infants appear to need REM to promote brain growth and development, adults still need REM to permit the brain changes required for learning and memory.  Studies in rats have shown both that if animals are deprived of REM sleep after learning a new task show less proficiency on the task the next day, while animals engaged in learning new tasks during the day spend proportionally more time in REM sleep that night.  Thus REM must be needed to promote learning and memory of new tasks.

 

In humans, REM deprivation doesn’t appear to affect learning much, although people who actively learn more during the day do spend more time in REM that night.

 

Regulation of sleep

Sleep is regulated by something since it happens in all warm-blooded mammals and the amount of time required in slow-wave and REM sleep is kept relatively constant—we subtract time from slow-wave sleep if we nap during the day, while we make up slow-wave and REM time if deprived for some time.   However, what regulates sleep is not clear.

 

Sleep is not likely regulated by anything in the blood since some dolphins & porpi sleep with only one hemisphere at a time—if the chemical was in the blood, then both hemispheres would be affected.

 

Because benzodiazapines, which are GABA-A agonists, promote sleep, GABA might be involved in sleep, except no one has been able to confirm this.

 

A different neurotransmitter though, adenosine, does appear to be involved in the promotion of sleep.  Adenosine is an inhibitory transmitter.  Caffeine is an antagonist at adenosine receptors.  Thus, caffeine can block adenosines effects on sleep, and thus keep us awake.

 

 

Biological Clocks and Circadian Rhythms

Most behavior follows regular rhythms.  Sleep cycles last 90 minutes, and daily patterns of sleep/wakefulness follow a 24-hour cycle.

 

Circadian rhythms describes our body’s natural 24-hour cycle.  This cycle is regulated by exposure to light/dark.  It appears to be controlled in the brain by the superchiasmatic nucleus (SCN) in the hypothalamus.  SCN also receives fibers from the retina, which allows it to remain in sync with light/dark schedules.  Certain ganglion cells produce their own chemical called melanopsin; these cells are sensitive to light, and send messages to SCN.

 

 

The pineal gland also helps to regulate circadian and seasonal rhythms.  The pineal gland gets input from the SCN during the night, and then secretes the hormone melatonin.   Melatonin feeds back to other brain structures including SCN to control hormone, phsyicological processes & seasonal behaviors like mating, etc.  During long nights (i.e., winter), more melatonin is secreted, so animals go into winter phase of cycles.

 

Shift work and jet lag effects on circadian rhythms

SCN tells brain it’s time to sleep, whether external cues match up or not…correct by trying to get external cues (“zeitgebers”) to match internal clock as soon as possible.