Hersenen en Gedrag – Samenvatting Deeltentamen 2
Chapter 8: Wakefulness and Sleep
Module 8.1: Rhythms of Waking and Sleeping
Endogenous Rhythms
Endogenous circannual rhythm: a rhythm generated from within that lasts about a year, like
birds that generate a rhythm that prepares them for seasonal changes.
Endogenous circadian rhythms: rhythms generated from within that last about a day, like the
rhythm of waking and sleeping. But also eating and drinking, urination, hormone secretion,
metabolism, sensitivity to drugs, and other variables like mood.
Humans generate 24-hour wake-sleep rhythms, which we can only modify a little.
Setting and Resetting the Biological Clock
Human’s circadian rhythms generate a period close to 24 hours, but they are not perfect. We
readjust our internal workings daily to stay in phase with the world. Sometimes, we misadjust
them. Unless something resets it from time to time, it would gradually drift away from the
correct time.
Zeitgeber (‘time-giver’): the stimulus that resets the circadian rhythm. The most dominant
zeitgeber for land animals is light, for some marine animals the tide is the most dominant.
Other zeitgebers include exercise, arousal of any kind, meals, and the temperature of the
environment. Social stimuli – the effects of other people – are ineffective as zeitgebers, unless
they induce exercise or other vigorous activity. Although these additional zeitgebers modify
the effects of light, they have only weak effects on their own.
Blind people who need to set their circadian rhythms by zeitgebers other than light set their
circadian rhythms by different zeitgebers, like noise, temperature, meals, and activity.
However, others who are not sufficiently sensitive to these secondary zeitgebers produce
circadian rhythms that are a little longer than 24 hours. This is no problem when their cycles
are in phase but when they drift out of phase, they experience insomnia at night and sleepiness
during the day.
Jet Lag
Jet lag: a disruption of circadian rhythms due to crossing time zones. Problems from jet lags
stem from the mismatch between internal circadian clock and external time.
Most people find it easier to adjust to crossing time zones going west than east. Going west,
we stay awake later at night and then awaken late the next morning, already partly adjusted to
the new schedule. We phase-delay our circadian rhythms. Going east, we phase-advance to
sleep earlier and awaken earlier. Most people find it difficult to go to sleep before their body’s
usual time and difficult to wake up early the next day.
Adjusting to jet lag is often stressful. Stress elevates blood levels of the adrenal hormone
cortisol, and many studies have shown that prolonged elevations of cortisol damage neurons
in the hippocampus, a brain area important for memory. Repeated adjustments of the
,circadian rhythm might bring danger with them, although the problem could also be air travel
itself.
Shift Work
People who sleep irregularly find that their duration of sleep depends on when they go to
sleep. When they have to sleep in the morning or early afternoon, they sleep only briefly, even
if they have been awake for many hours. People who have done shift work for years tend to
perform worse than average on cognitive tests, although because the measures are
correlational, we cannot be sure of cause and effect.
People adjust best to night work if they sleep in a very dark room during the day and work
under very bright lights at night, comparable to the noonday sun. Short-wavelength (bluish)
light helps to reset the circadian rhythm better than long-wavelength light does.
Morning People and Evening People
Circadian rhythms differ among individuals. But many people are intermediate between the
two extremes of evening and morning people.
Children almost always go to bed early and wake up early. When entering adolescence,
people usually start staying up later and waking up later, when having the opportunity. The
mean preferred time of going to sleep gets later and later until about age 20 and then gradually
reverses.
So, being a morning person or an evening person depends partly on age. It also depends on
genetics and several environmental factors, including artificial light.
The tendency for most young people to be evening types causes problems, because the
environment is not adapted to this. Even beyond the teenage years, morning people report
being happier than evening people, on average, possibly because their biological rhythms are
more in tune with their 9-to-5 work schedule.
Mechanisms of the Biological Clock
The biological clock is a hardy, robust mechanism and is insensitive to most forms of
interference.
The Suprachiasmatic Nucleus (SCN)
Suprachiasmatic nucleus (SCN): the main driver of rhythms for sleep and body temperature.
It is a part of the hypothalamus.
The SCN generates circadian rhythms itself in a genetically controlled manner. Even a single
isolated SCN cell can maintain a circadian rhythm, although interactions among cells sharpen
the accuracy of the rhythm. The rhythms come from the SCN itself.
How Light Resets the SCN
A small branch of the optic nerve, known as the retinohypothalamic path, from the retina to
the SCN, alters the SCN’s settings.
Most of the input to that path does not come from normal retinal receptors, but from a special
population of retinal ganglion cells that have their own photopigment, called melanopsin,
,unlike the ones found in (normal) rods and cones. These special ganglion cells receive some
input from rods and cones, but even if they do not receive that input, they respond directly to
light. But they respond to the overall average among of light, not to instantaneous changes in
light. This average intensity over a period of time is exactly the information the SCN needs to
gauge the time of the day. These ganglion cells respond mainly to short-wavelength (blue)
light. For that reason, exposure to television, video games, computers, and so forth, all of
which emit mostly short-wavelength light, tends to phase-delay the circadian rhythm and
make it difficult to fall asleep at the usual time. (Thus, short-wavelength light in the morning
or during the night can help people working night/early shifts to stay awake because it makes
their bodies feel like it’s daytime, while short-wavelength light in the evening when you
should be getting ready to sleep has the effect that you can’t fall asleep because your body
still thinks it’s daytime).
The Biochemistry of the Circadian Rhythm
Studies of the fruit fly Drosophila found several genes responsible for a circadian rhythm.
Two of these genes, known as period (abbreviated PER) and timeless (TIM), produce the
proteins PER and TIM. The concentration of these two proteins, which promote sleep and
inactivity, oscillates over a day, based on feedback interactions among neurons. Early in the
morning, the messenger RNA levels responsible for producing PER and TIM start at low
concentrations. As they increase during the day, they increase synthesis of the proteins, but
the process takes time, and so the protein concentrations lag hours behind. As the PER and
TIM protein concentrations increase, they feed back to inhibit the genes that produce the
messenger RNA molecules. Thus, during the night, the PER and TIM concentrations are high,
but the messenger RNA concentrations are declining. By the next morning, PER and TIM
protein levels are low, the flies awaken, and the cycle is ready to start again. because the
feedback cycle takes about 24 hours, the flies generate a circadian rhythm even in an
unchanging environment. However, in addition to the automatic feedback, light activates a
chemical that breaks down the TIM protein, thereby increasing wakefulness and
synchronizing the internal clock to the external world.
Mammals have three versions of the PER protein and several proteins closely related to TIM
and the others found in flies.
Melatonin
The SCN regulates waking and sleeping by controlling activity levels in other brain areas,
including the pineal gland (= pijnappelklier). The pineal gland releases the hormone
melatonin.
In diurnal animals like humans, it increases sleepiness. In nocturnal animals, it increases
wakefulness. Melatonin also helps control the onset of puberty and bodily adjustments to
changes of season (such as hibernation).
Module 8.2: Stages of Sleep and Brain Mechanisms
Sleep and Other Interruptions of Consciousness
, Sleep is a state that the brain actively produces, characterized by decreased activity and
decreased response to stimuli.
Coma: an extended period of unconsciousness caused by head trauma, stroke or disease.
Someone in a coma has a low level of brain activity and little or no response to stimuli.
Typically, they either die or begin to recover within a few weeks.
Vegetative state: someone who alternates between periods of sleep and moderate arousal,
although even during the moderate aroused stage, the person shows no awareness of
surroundings and no purposeful behavior.
Minimally conscious state: one stage higher than vegetative state, with brief periods of
purposeful actions and a limited amount of speech comprehension.
Brain death: condition with no sign of brain activity and no response to any stimulus.
The Stages of Sleep
An EEG record rises or falls when most cells do the same thing at the same time – look at
general activity, not activity of particular neurons.
Polysomnograph: a combination of EEG and eye-movement records
Relaxed, awake: alpha waves at a frequency of 8 to 12 per second. Characteristic of
relaxation, not of all wakefulness. – fast waves.
Stage 1 sleep (not always seen as sleep): EEG is dominated by irregular, jagged, low-voltage
waves. Brain activity is less than in relaxed wakefulness but higher than in other sleep stages.
Stage 2: the prominent characteristics are K-complexes and sleep spindles. A K-complex is a
sharp wave associated with temporary inhibition of neuronal firing. A sleep spindle is a burst
of 12- to 14-Hz waves for at least half a second (short, fast waves).
Sleep spindles increase in number after new learning, and the number of sleep spindles
correlates positively with improvements in certain types of memory. The sleep spindles
represent activity related to the consolidation of memory.
Slow-wave sleep: heart rate, breathing rate, and brain activity decrease, whereas slow, large-
amplitude waves become more common. Slow waves indicate that neuronal activity is highly
synchronized.
Paradoxical or REM sleep
A scientist noticed that in certain periods of apparent sleep, a cat’s brain activity was
relatively high, but their neck muscles were completely relaxed and called this paradoxical
sleep because it is deep sleep in some ways and light in others.
Rapid eye movement (REM) sleep: periods in which rapid eye movements occur during sleep.
Synonymous with paradoxical sleep but paradoxical sleep is rarely used for humans.
Non-REM (NREM) sleep: stages other than REM.
During paradoxical or REM sleep, the EEG shows irregular, low-voltage fast waves that
indicate increased neuronal activity. In this regard, REM sleep is light, and similar to stage 1
