PL3232 Ch8.1 Rhythms of Waking and Sleeping

Jet Lag

= A disruption of circadian rhythms due to crossing time zones. Basically, a mismatch between internal circadian rhythm and external time. Symptoms: Sleepiness during the day, sleeplessness at night, impaired concentration. Travel WEST = Phase Delay (Sleep later, wake later). Travel EAST = Phase Advance (Sleep earlier, wake earlier). Travelling EAST creates jet lag. Adjusting to jet lag is stressful: Stress elevates cortisol levels in blood. Prolonged elevations of cortisol damages the hippocampus. This leads to a suspected danger that repeated adjustments of the circadian rhythm is dangerous.

Endogenous Circadian Rhythm

A cycle that lasts about a day. Light from the sun helps you feel less sleepy, but the urge to sleep depends on the time of the day, not just how long you have been awake. It occurs even in unchanging total darkness. It is a self-generated cycle that is about 24 hours, varying across individuals. People can adjust their schedules fairly easily, but if it is a lot longer than 24 hours, it is difficult.

Morning people and Evening people

Circadian rhythms differ in people. Mean preferred time to sleep gets later until about 20 years old, then it reverses gradually. This is reflected in every culture: Adolescents prefer later sleep and later waking up. This also occur in other animals. One explanation is that it is due to the increased levels of sex hormones. So: Morning or Evening person depends on age, genetics and other factors. Living in cities = more light = more likely to stay up. Evening people may suffer from 'social jet lag' - suffering from activities that delay sleep, but have to wake up early. Morning people report being happier than evening people across ages, perhaps because their circadian rhythm is in tune with the socioeconomic requirements of 9-5.

Endogenous Rhythms

Endogenous Circannual Rhythm: Birds generate a rhythm that prepares them for seasonal changes. Endogenous Circadian Rhythm: A cycle that lasts about a day.

Setting and Resetting the Biological Clock

Human circadian rhythm is about 24 hours. We readjust our internal workings daily to stay in phase changes in the world. Circadian Rhythms are biological - they persist without light. However, light is a crucial ingredient in resetting it. Factors that reset the circadian rhythm are called "Zeitgebers". People who were already sleep deprived have difficulties in adjustment.

Mechanisms of the Biological Clock

It is typically insensitive to most forms of interference - even the blind and deaf have circadian rhythms, even if they drift out of phase. It remains steady despite food/water deprivation, x-rays, alcohol, anesthesia, lack of oxygen, brain damage, removal of endocrine organs.

Melanopsin Ganglion Cells

Melanopsin = a special population of retinal ganglion cells have their own photopigment. They are different from rods and cones. Melanopsin ganglion cells receive some inputs from rods and cones but don't rely on them. They respond directly to light. They are located near the nose, where they look towards the periphery. They respond to the overall average amount of light, not to instant changes in light. They respond slowly and turn off slowly when the light ceases. They use the average intensity over a period of time to gauge the time of the day, and respond mainly to short-wavelength (blue) light. Consequences of having Melanopsin: 1) Blind people don't have to rely on damaged rods, cones and/or visual cortex. Melanopsion ganglion cells can entrain their wake/sleep cycles. 2) Bright light aggravates headache for blind people. Melanopsin cells send input to the posterior thalamus, which is part of the pain producing pathways for migraines. 3) They respond strongly to short-wave light. Exposure to such light late in the day tends to reset the circadian rhythm and phase-delaying it. Short-wave lights are all around us: TV, PC, phones.

Effects of the Circadian Rhythm

More than just waking and sleeping: Eating, drinking, urination, hormone secretion, metabolism, sensitivity to drugs, etc. Moods also have circadian rhythms: Increases in positive mood (happiness) from waking til late afternoon, then slight decline until bedtime. Hence, there seems to be a biologically driven circadian rhythm in our emotional well-being.

Biochemistry of the Circadian Rhythm

SCN functions by the work of genes. IN INSECTS, there are 2 proteins: PER and TIM. PER = Period. TIM = Timeless. They promote sleep and inactivity. Their concentration oscillates over a day based on feedback interactions among neurons. In the morning, PER and TIM concentrations are low, and gradualy increase due to protein synthesis. This results in inhibition of the genes that produce the messenger RNA molecules. At night, PER and TIM concentrations are high but the messenger RNA concentrations are declining. By the next morning, PER and TIM protein levels are low, and the flies awaken. Light also breaks down the TIM protein, thereby increasing wakefulness and synchronizing internal and external clocks. IN MAMMALS, we have 3 versions of the PER protein and several proteins related to TIM. Particular mutations of PER result in circadian rhythms shorter than 24 hours. They get sleepy early and awaken early. They like to go to bed. Other mutations reduce the amount of sleep people need, or impair people's ability to rebound from temporary sleep deprivation.

How does light reset the SCN?

SCN is located just above the optical chiasm. Retinohypothalamic path = a small branch of the optic nerve from the retina to the SCN. The retinophypothalamic path alters the SCN's settings. Most of the input to this path does not come from normal retinal receptors - it comes from Melanopsin ganglion cells.

Melatonin

SCN regulates waking and sleeping by controlling activity levels in other brain areas, including the pineal gland. Pineal gland = release melatonin. Melatonin = hormone that influences both circadian and circannual rhythms. The pineal gland secretes melatonin mostly at night, making us sleepy. Secretion starts to increase about 2-3 hours before bedtime. Melatonin pills are used to help readjust circadian rhythms.

Shift Work

Some occupations require workers to sleep irregularly (pilots, shift workers). Duration of sleep depends on where they go. People do not completely and properly adjust to night shift work. Even after years of night shift work, workers continue to feel groggy and sleep poorly in the day. Body temperature continues to peak in the day. Most buildings use artificial lighting from 150-180 lux - only moderately effective in resetting the rhythm. People best adjust if they sleep in a very dark room and work in a very bright place that is similar to the noon sun.

SCN: Suprachiasmatic Nucleus

The main driver of rhythms for sleep and body temperature. Damage to the SCN = erratic body rhythms. SCN is part of the Hypothalamus. Rhythms of the SCN are genetically controlled. Even if there is just 1 SCN cell, it can maintain a circadian rhythm. Interactions with other cells sharpen the accuracy of the rhythm. SCN also regulates waking and sleeping by controlling activity levels in other brain areas such as the pineal gland.

Zeitgeber

The stimulus that resets the circadian rhythm. "Time-giver". Light is the dominant zeitgeber for land animals. Other zeitgebers: Exercise, Arousal (of any kind), meals, temperature of the environment. Social stimuli are ineffective as zeitgebers unless they induce exercise or other vigorous activities. What about the blind? They use secondary zeitgebers (noise, temperature, meals and activity). The blind who are not sufficiently sensitive to secondary zeitgebers produce circadian rhythms longer than 24 hours.