Sleep Stages

• Stages of sleep correspond to massive electrophysiological changes that occur throughout the  brain  as  the  fast,  irregular,  and  low  amplitude  electrical activity  of  wakefulness  is replaced by slow, regular, high amplitude waves of deep sleep.
• Using the electroencephalograph (EEG) to monitor the brain’s electrical activity during waking and  sleep  states,  scientists  have  discovered  5  distinct  stages of  sleep  and two stages of wakefulness.
• When people are wake, their EEG patterns exhibit two types of waves: beta and alpha.
• Beta waves reflect wakefulness. These waves are the highest in frequency and lowest in amplitude. They also are more Desynchronous than other waves. Desynchronous waves do not form a very consistentpattern.  Inconsistent patterning  makes  sense  given  the extensive  variation  in  sensory  input  and activities  we  experience  when  we  are  awake. When we are relaxed but still awake, our brain waves slow down, increase in amplitude and become more synchronous, or regular. These waves are called alpha waves

The depth of sleep varies from one stage to another:

• Stage  1  sleep  is  characterized  by  theta  waves,  which  are even slower  in frequency and greater in amplitude than alpha waves.
• Stage 2 sleep is when theta waves continue but are interspersed with a defining characteristic of stage 2 sleep: sleep spindles. These involve a sudden increase in wave frequency. Stages 1 and 2 are both relatively light stages of sleep, and if people awaken during these stages, they often report not having been asleep at all.
• Stage  3  and  4  are  characterized  by  delta  waves,  the slowest  and  highest amplitude brain  waves during sleep. Distinguishing between stage 3  and  4  isdifficult,  although typically stage  3  is characterized  by delta waves occurring less than 50% of the time and stage 4 is characterized by delta waves occurring more than 50% of the time. Delta sleep is our deepest sleep, the time when ourbrain waves are least waking brain waves.

REM Sleep

• After going  through  stages  1  through  4,  sleepers  drift  up through  the  sleep stages toward wakefulness. But, instead of re-entering stage 1, they enter stage5, a different form of sleep called REM (Rapid-eye-movement) sleep.
• REM sleep is an active stage of sleep during which dreaming occurs
• During REM  sleep,  the  EEG  pattern  shows  fast  waves similar  to  those  of relaxed wakefulness, and the sleeper’s eyeballs move up and down from left to right.
• Stages 1-4 are referred to as non-REM sleep à Characterized by a lack of REM and little dreaming.
• Even people who claim they rarely dream frequently report dreaming when they are awakened during REM sleep. The longer the period of REMsleep, the more likely it is that the person will report dreaming.
• These dramatic developmental changes in sleep, especially REM sleep, raise questions about the function of sleep.
• REM sleep may be nature’s way of stimulating the brain.
• REM sleep likely also contributes to memory.

Sleep Cycling Through the Night

• The 5 stages of sleep described here make up a normal cycle of sleep.
• The amount of deep sleep is much greater in the first half of a night’s sleep than the second half.
• Most REM sleep takes place toward the end of night’s sleep, when the REMstage becomes progressively longer.

Sleep and the Brain

• The 5  sleep  stages  are  associated  with  distinct  patterns  of neurotransmitter activity initiated in the reticular formation, the core of the brain stem.
• Damage to the reticular formation can result in coma and death.
• Important neurotransmitters involved in sleep are serotonin, epinephrine, and acetylcholine.
• As sleep begins, the levels of neurotransmitters sent to the forebrain from the reticular formation start  dropping,  and they  continue  to  fall  until  they  reach their lowest levels during the deepest sleep stage—stage 4.
• REM sleep is initiated by a rise in acetylcholine, which activates the cerebral cortex while the rest of the brain remains relatively inactive.  REM sleep is terminated by a rise in serotonin and norepinephrine, which increase the level of forebrain activity nearly to the awakened state. You are most likely to wake up   just   after   a   REM   period.   If   you don’t   wake   up   then, the   level of neurotransmitters begins to fall again, and you enter another sleep cycle.