Sleep
Fall 2002

Humans spend approximately 1/3 of life asleep (i.e., 8 hours a day).

A common misconception is that sleep is a passive loss of consciousness. However, an electroencephalogram reveals that humans cycle through several stages of brain activity every 90 minutes (an EEG measures the summed activity of pyramidal neurons in the cerebral cortex). These recordings reveal two phases during sleep:

1. Slow-wave sleep is characterized by slow synchronized brain waves indicative of a relatively inactive brain.
    Slow-wave sleep has four stages (Figure 17.10):
Stage 1: enter only when you begin to sleep. (theta waves)
Stage 2: a brief stage characterized by activity spindles and K complexes.
Stage 3 & 4: also called deep sleep. Characterized by large slow waves. Deep sleep disappears as the night progresses.   2. REM sleep for Rapid Eye Movements (also called paradoxical sleep). Replaces stage 1 during cycle. REM sleep increases as the night progresses. REM sleep is characterized by: a) rapid eye movements
b) dreams
c) paralysis of skeletal muscles
d) PGO waves (brain waves that travel from the Pons to the Geniculate nucleus to the Occipital cortex).
e) desynchronized brain waves as if awake.
  There are two hypotheses that attempt to explain why animals sleep: 1. Restoration theory states that sleep allows the brain time to repair itself and/or consolidate memories.

2) Adaptation theory states that sleep evolved as a way to conserve energy and avoid predators. This theory is based on the finding that predators eat once a day and sleep a lot to conserve energy, whereas grazers eat all day and sleep less than four hours a day.
 
 

 The human body maintains three simultaneous cycles:
1. 90 minute cycle
2. Circadian cycle
3. Seasonal cycle
Sleep is part of the body's daily cycle. Other functions that cycle include eating & drinking, body temperature, hormones, and urination. Daily cycles are called circadian rhythms. Everyone has an internal clock that tells you when to do things. The sleep cycle continues even in a constant environment with no day/night cues (The human cycle actually runs about 25, not 24 hours). The biological clock that maintains this cycle is located in the suprachiasmatic nucleus in the hypothalamus. Neurons in the suprachiasmatic nucleus have two important features: 1. An activity rhythm occurs even when neurons are removed from the brain.
2. Input from the retina resets the clock each day.
  A person's arousal level is closely linked to two brain structures: 1. Locus coeruleus (LC) neurons release norepinephrine (NE) throughout the brain. Stimulation of the LC increases arousal.

2. Raphe nuclei release serotonin throughout the brain. Neurons in the dorsal raphe nucleus (midbrain) are active when animals are awake and quiet when animals are asleep.

 A chemical called adenosine appears to be able to trigger sleep. Adenosine is a by-product of ATP which is used for energy in cells. Thus, active cells have higher levels of adenosine. High adenosine levels in the basal forebrain is associated with sleep. Caffeine keeps you awake by blocking adenosine.
 
 

Some common sleep disorders include:

Insomnia: the most common sleep disorder. Sleeping pills are a temporary solution that will exacerbate insomnia in the long run.
Narcolepsy: a sleep attack whereby someone goes instantly from awake to REM sleep.
Cataplexy: paralysis associated with REM sleep despite being awake.
REM sleep behavior disorder: act out dreams because muscles are not paralyzed.
Somnabulism: sleep walking during slow-wave sleep. Common in children, but rare in adults.
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