Chapter 6: Sleep

In this Chapter:

• Brain Activity during Sleep
• Sleep disorders
• How is sleep regulated?
• The Sleep-Wakefulness Cycle

Introduction

• Lack of sleep increases the risk of many health problems including:
  • Diabetes
  • Cardiovascular disease
  • Heart attacks
  • Stroke
  • Depression
  • High Blood Pressure/Hypertension
  • Obesity
  • Infections
• Sleep is made of several different stages that are accompanied by daily rhythms in hormones, body temperature, etc.
• Sleep disorders are one of the least recognized sources of disease, disability, and death

Brain Activity During Sleep

• Electroencephalography (EEG): the measurement of electrical activity in different parts of the brain and the recording of such activity as a visual trace (on paper or on an oscilloscope screen).
• One sleep cycle is approximately 90 minutes
• Each night, the brain progresses through a series of stages when brain waves slow down for 60 minutes in NREM (non-rapid eye movement) sleep
  • This is accompanied by the relaxation of muscles and eyes
  • Heart rate, Blood pressure, and body temperature fall
  • If awakened in this stage of sleep, most people only recall fragments of thought
  • NREM sleep: sleep where the eyes do not rapidly move
• Over the next half-hour, brain activity changes to REM sleep
  • REM (Rapid Eye Movement) sleep: sleep characterized by the random rapid movement of the eyes when derams also occur
  • Characterized by neocortical (in the neocortex) EEG waves similar to those when a person is awake
  • REM is accompanied by atonia
  • Atonia: paralysis of muscle
  • Dreaming occurs only in REM sleep
  • The first REM period lasts 10-15 min
• Over the course of one’s lifetime: slow-wave sleep time decreases & REM time increases
• Overall sleeping time at different life stages:
  • Infants: up to 18 hrs
  • Older adults: 6-7 hrs
  • Less time sleeping in general and in slow wave sleep specifically

Sleep Disorders

• Insomnia: the most common sleep disorder where individuals have trouble falling asleep
  • Some people have problems falling asleep, some wake up in the middle of the night and can’t fall asleep again
  • Sleeping drugs do not help because they suppress slow-wave sleep and aren’t effective in keeping people asleep
• Many common disorders disrupt deeper stages of sleep
• Excessive daytime sleepiness has many causes
  • Obstructive Sleep Apnea: airway muscles relax and close airway causing difficulty breathing
  • individual wakes up without entering deeper stages of slow-wave sleep.
  • Causes high blood pressure and increases the risk of heart attack
  • more daytime sleepiness
  • Periodic Limb Movements: intermittent jerks of the legs or arms that occur as individual enters slow wave sleep and cause arousal from sleep.
  • REM behavior disorder: occurs when muscles fail to become paralyzed during REM sleep
  • Act out dreams by getting up and moving around.
  • Can be very disruptive.
  • Both periodic limb movements and REM behavior disorder are more common in people with Parkinson’s disease.
  • Can be treated with drugs for Parkinson’s or with a benzodiazepine called clonazepam
• Narcolepsy: mechanisms controlling transitions into sleep (particularly REM sleep) don’t work.
  • Narcolepsy is caused by the loss of nerve cells in lateral hypothalamus that contain orexin/hypocretin.
  • Have sleep attacks during day (suddenly fall asleep).
  • Hypnagogic hallucination: individuals tend to enter REM sleep very quickly and enter dreaming state while partially awake
  • Cataplexy: loss of muscle tone similar to what happens in REM sleep but occurs when the individual is still awake

How is Sleep Regulated?

• Wakefulness is maintained by systems in the upper brainstem and hypothalamus
  • Neurons here use acetylcholine, norepinephrine, serotonin, glutamate to connect with the forebrain
  • Neurons in the hypothalamus use orexin and some contain histamine
  • Thalamus and basal forebrain activation by acetylcholine is very important too
  • basal means “closest to midbrain/base”
• Level of alertness can be shown in an activated low-volt EEG
• Arousing systems are less active in non-REM sleep
  • Transmission of information to the thalamus is limited
• Ventrolateral preoptic (VLPO) nucleus: area in the brain that causes suppression of arousal systems
  • VLPO nucleus neurons have the inhibitors galanin and GABA
  • Damage to the VLPO area produces irreversible insomnia
• In REM sleep, there is an internally activated brain and EEG but the external input is suppressed
  • Internal activation comes from cyclically active REM sleep generator neurons in the brainstem
• Signals from neurons cause the excitation of the forebrain
  • Leads to rapid eye movements & muscle suppression
• Forebrain excitation driving force behind dreams of REM sleep
• Motor cortex neurons fire as rapidly during REM sleep as during waking movement
  • Explains movement coinciding with dreams
• Periodic recurrence of REM sleep
  • REM sleep occurs every 90 min during sleep
  • This is caused by on-and-off switching of REM-generators (acetylcholine, glutamate) and REM-suppressors (norepinephrine, serotonin, GABA)

The Sleep-Wakefulness Cycle

• 2 determining factors for sleepiness:
  • Circadian system: Monitoring the time of day/night
  • Homeostatic system: monitoring how long the person is awake
• Circadian system is regulated by the suprachiasmatic nucleus
  • Suprachiasmatic nucleus: a small group of cells in the hypothalamus serving as the master clock
  • It expresses clock proteins that go through a biochemical cycle of approximately 24 hrs
  • This sets the pace for daily cycles of activity, sleep hormone release, etc.
• The suprachiasmatic nucleus also receives input from the retina
  • The clock can be reset by light so it is linked to the outside day-night cycle
• Also provides information to the subparaventricular nucleus → dorsomedial nucleus → VLPO and orexin neurons
  • Orexin: an excitatory signal to arousal system especially norepinephrine neurons
  • Orexin activation plays role in preventing transitions into REM sleep during the day
• Arousal mediated by orexin and activation of norepinephrine neurons in the locus coeruleus
  • Locus coeruleus: a lateral part of brain stem that has norepinephrine producing neurons that mediate arousal along with orexin
• The homeostatic system responds to longer wake periods by increasing the urge to sleep
  • The longer a person is awake, the greater the likelihood of an increase in sleep-inducing factors
• Adenosine: a very important sleep promotor
  • More adenosine means increased sleepiness
  • Adenosine release starts in the basal forebrain and spreads to the rest of the cortex
  • Increased adenosine levels slow down cellular activity and diminish arousal
  • Adenosine levels decrease during sleep
• Brain adenosine may be produced by ATP breakdown over the course of wakefulness
• Neuron activity decreases and adenosine levels decline in non-REM sleep
  • ATP levels increase during sleep