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8. Wakefulness and sleepRhythms of waking and sleeping
Endogenous rhythms
Animals often need to anticipate changes in the environment, being ready for a change comes partly
from internal mechanisms. Endogenous circannual rhythm: prepares animals for seasonal
changes.
Endogenous circadian rhythm: last about a day. Affect waking and sleeping, eating, drinking,
hormones, peeing, metabolism, sensitivity to drugs,,,
When you don't sleep all night, you will feel more sleepy as the night goes on but in the morning you
will feel more alert. Activity (mainly) in the posterior areas of the cerebral cortex correlates mostly with
your circadian rhythm, less with how long you have been awake.
The circadian rhythm does not easily adjust from severe departures from a 24-hour schedule.
Setting and resetting the biological clock
The circadian rhythm generates a period close to 24 hours, but is adjusted everyday to stay in phase
of the world. Sometimes they are misadjusted, for example during weekends exposed to light and
activity at night, the biological clock may think it is earlier than it is.
Without light the circadian rhythm would persist, but would gradually drift away from the correct time if
not resetted. Zeitgeber: time-giver, a stimulus that resets the circadian rhythm. Light is the dominant
zeitgeber for land animals, others are arousal, food, temperature,,, Social stimuli are only effective if
they cause strong activity. These other zeitgebers have a weak effect on their own, but can modify the
effects of light.
Some blind people need to set their rhythms by other zeitgebers. Or their circadian rhythms are a little
longer than 24 hours, when they drift out of phase with the clock they get insomnia and sleepiness
during the day.
Jet lag: disruption of circadian rhythms due crossing time zones. Causes sleepiness during the day,
sleeplessness at night, depression and bad concentration. Going west is easier, staying awake later
and then waking later in the morning: phase delay, going east is earlier: phase advance.
Adjusting can be stressful, stress elevates the blood levels of cortisol. Prolonged elevations of cortisol
damage neurons in the hippocampus, brain area important for memory.
For people who sleep irregularly, the duration of their sleep depends on when they go to sleep. In the
morning or early afternoon they sleep short, even after being awake for a long time. Even after years
many workers don't completely adjust. Morning people are most impaired working the night shift.
People adjust best if they sleep in a dark room and work under very bright lights at night. Short
wavelength light helps to reset the circadian rhythm better than long wavelength light.
Circadian rhythms differ among individuals, most people are in between the two extremes. The
preferred time of going to bed gets later until age 20, then gradually reverses.
Mechanisms of the biological clock
In 1967 Cur Richter introduced the concept that the brain
generates its own rhythms, a biological clock. He reported
that the clock is insensitive to most forms of interference.
,The main driver of rhythms for sleep and body temperature is the suprachiasmatic nucleus (SCN),
it's part of the hypothalamus. It generates the rhythm in a genetically controlled manner.
The retinohypothalamic path is a small branch of the optic nerve from the retina to the SCN, it alters
the SCN. The input comes from a special group of retinal ganglion cells which have their own
photopigments: melanopsin. These cells receive some input from rods and cones, but mostly respond
directly to light. They slowly turn off when light stops. The average intensity of light is the information
the SCN needs to measure the time of the day. These ganglion cells mainly respond to short
wavelength (blue) light. Exposure to blue light tends to phase delay the circadian rhythm.
There are several genes responsible for a circadian rhythm. Period (PER) and timeless (TIM) are
genes that produce the proteins PER and TIM, which promote sleep and inactivity. The concentration
of these proteins fluctuates, based on feedback interactions among neurons. mRNA levels
responsible for producing PER and TIM are low in the morning, increases during the day. Creating
synthesis of the proteins takes time, so the protein concentration lays behind. As PER and TIM
increase, they feedback to inhibit the genes that produce the mRNA.
So during the night PER and TIM concentrations are high, by the next morning the levels are low and
the cycle starts again.
Light activates a chemical that breaks down the TIM protein, increasing wakefulness and
synchronizing the internal clock to the external world.
The SCN regulates waking and sleeping by controlling activity in other brain areas.
The pineal gland: an endocrine gland, posterior to the thalamus. It releases the hormone melatonin:
chemical released mostly at night, regulating sleep and wakefulness, increases sleepiness. In
nocturnal animals it increases wakefulness. Melatonin also helps control onset puberty and body
adjustments to changes of season. Melatonin release increases 2/3 hours before bedtime.
Stages of sleep and brain mechanisms
Sleep and other interruptions of consciousness
Sleep is a state that the brain actively produces, decreased activity and response to stimuli.
Coma: extended period of unconsciousness caused by head trauma or disease, low level of brain
activity and little to no response to stimuli.
Vegetative state: alternation between periods of sleep and moderate arousal, but even during
aroused state the person shows no awareness of surroundings and no purposeful behavior. Breathing
is more regulated, pain produces at least autonomic responses.
Minimally conscious state: brief periods of purposeful actions, limited amount of speech
understanding.
Brain death: no sign of brain activity and no response to any stimulus.
Stages of sleep
Electroencephalograph (EEG): records electrical activity of the brain through electrodes attached to
the scalp. The electrodes measure the average of electrical potentials of cells and fibers in the brain
area nearest to each electrode.
If half the cells increase and half decreases, it cancels out.
Polysomnograph is a combination of EEG and eye-movement records.
, Aplha waves are characteristic of relaxation, not of all wakefulness.
Stage 1 sleep (b) is the beginning of sleep, in this stage the EEG has
irregular jagged low-voltage waves. Brain activity is less than when
awake, but higher than in other sleep stages.
Stage 2 sleep (c) has K-complexes: sharp waves associated with
temporary inhibition of neuronal firing and sleep spindles: burst of 12-
14Hz waves for at least half a second. Sleep spindles result from back
and forth interactions between cells in the thalamus and the cortex,
they increase after new learning, more spindles go with improvement
of certain types of memory.
During slow-wave sleep (d, e) the heart rate, breathing rate and brain activity decrease. Slow large-
amplitude waves increase more and more. Slow waves indicate that neuronal activity is highly
synchronized, most cells are synchronized. Input to the cerebral cortex is inhibited.
Paradoxical/REM sleep
In the 1950s Michel Jouvet wanted to test learning abilities of cats after removing the cerebral cortex.
He only recorded slight movements of the muscles and EEGs from the hindbrain. He noticed that
during certain periods of sleep the brain activity was relatively high but their neck muscles were
relaxed. He then recorded the same in normal cats and called it paradoxical sleep: the sleep is deep
in some ways and light in other ways.
Nathaniel Kleitman and Eugene Aserinsky observed eye movement to measure the onset and offset
of sleep. After repeated measurements they concluded that periods of rapid eye movement occur
during sleep: rapid eye movement (REM) sleep. This sleep is the same as the paradoxical sleep.
During this stage the EEG shows irregular low-voltage fast waves, indicating increased neuronal
activity. Postural muscles are more relaxed than during other stages. Heart rate, blood pressure,
breathing rate and facial twitches fluctuate more during REM.
Other stages are called non-REM (NREM) sleep.
After about an hour you go from sws back to stage 2, then REM. Each cycle lasts about 90 minutes.
Early in the night sws dominates, then the time REM occupies increases. The amount of REM
depends more on the time of the day than how long you have been asleep.
The pattern of sleep stages varies per age, health and other factors.
The frequency of waking correlates with loss of cells in the hypothalamus, tending towards cognitive
decline.
REM dreams are more likely to include visual images and complicated plots than NREM dreams.
Brain mechanisms of wakefulness, arousal and sleep
Animals need to regulate the level of alertness. Many brain areas help control of sleep and
wakefulness.
Damaging the reticular formation: a structure that extends from the medulla into the forebrain,
decreases arousal in the midbrain. Some neurons in the reticular formation have axons going into the
brain and some going to the spinal cord. The neurons with axons into the spinal cord form part of the
media tract of motor control.
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