During a night’s sleep you pass through different sleep cycles of 90 mins each with variance in amount of the different stages. Difference in stages is not just oscillations of depth but qualitatively different.Sleep stages alternate throughout night starting w/ rapid descent into deep sleep. Most complete 5 ultradian cycle w/ progressively less SWS & more REM towards morning Stage 1 – ‘drift off’ relaxed state. Brain waves change from beta (awake) to alpha: greater amplitude, slower and partly irregular.
Easy to wake in this stage Stage 2 – slower, larger brain waves & occasionally quick bursts of high frequency sleep spindles + K-complexes Stage 3 – beginning of deep sleep. Brain waves are slow and large, called delta w/ some sleep spindles. Slowing down of bodily activity Stage 4 SWS – deepest sleep. Delta waves are slow, large and show jagged pattern on EEG. Difficult to wake and bodily functions sink to deepest state of physical rest. EMG shows muscles as relaxed. Sleep disorders + growth hormone production are associated.
REM (paradoxical sleep) – similar signs of waking on EEG (why it’s paradoxical) muscles completely relaxed but eye movement is rapid. End of REM marks end of first ultradian rhythm Dement – REM associated with dreaming through EEG, all people dream (those who don’t just forget). 80% of pps woken during REM reported dreams in vivid detail. Only 7% of awakening from NREM led to dream recall. Dreams don’t last longer than 15 mins, beyond this time people forget the beginning. Content can be affected by events experienced during the day & external stimuli
Reticular formation, locus coeruleus & raphe nuclei control REM / NREM (links w/ affective disorders & sleep) Evaluation However, REM & dreaming v. different. REM = physiological state objectively measured. Dreams = subjective + problems studying scientifically Some studies reported 70% dreams in NREM (attributed to confusion of what is dreaming) Physiology of sleep Brain stem controls arousal and + sleep. Oldest part of reptilian brain, which controls vital functions Serotonin – SCN responds to changes in light & controls production of melatonin in pineal land –> production of serotonin in raphe nuclei. Increase of serotonin reduces RAS activity = onset of sleep.
Noradrenalin – from locus coeruleus (part of pons) triggers REM Functions of sleep Evolutionary explanations Sleep patterns evolved as an adaption. Each species has developed different sleep habits and patterns to deal w/ environment pressures e.g. predation, energy and foraging. Main assumption is that animal’s sleep pattern will be determined by its energy intake & expenditure pattern, concentrates on ecological niches.
1. Energy conservation / hibernation theory -mammals expend a lot of energy on body temperature, particularly evident in small animals w/ high metabolism. Conserving energy at times of little resources (night) + Sleep increases when energy loss goes up (cold) 2. Foraging requirements – Sleep is related to amount of time need for finding food. Herbivores require grazing and therefore cannot afford to sleep as much as carnivores (links with the predation theory + lower metabolism supports energy conservation) 3. Protection from predation – Being awake is riskier than being asleep, it ensures that animals stay still (however can increase vulnerability)
Evaluation
Helped understanding of animals but incomplete in amount of sleep stages. NREM may be functional for energy conservation compared to REM (research shows correlation between animal size and NREM but not REM). New hypothesis based on mammalian brain of reptile not evolving REM, but no research support Can’t apply to humans (may be more beneficial not to sleep now – genome lag) Comparative studies generally supports view e.g. small animals w/ high metabolism sleep more, but there are anomalies such as the sloth (energy) and rabbit (predation)
Universal concept, species differ in sleep pattern e.g. unilateral sleeping support adaptation Behavioural inactivity would be a better function + snoring = vulnerability bad explanation Capellini – previous research flawed in methods of non-standardisation (supported in phylogeny sleep project). Found neg correlation between metabolism rate and sleep. This falsifies energy conservation but supports trade off view of foraging requirements. Also found no correlation between predation and sleep Fails to address restoration findings, combined approach of all 3 + restoration is supported by Horne’s core vs. optional
Restoration theory -Oswald Sleep allows various physiological + psychological states to be recovered e.g. infancy need for brain & body growth + learning / memory consolidation Physiological restoration SWS – immune system + growth hormone correlation & neural control (lifespan differences) REM – brain growth + infant neuroplasticity + correlation with brain maturity e.g. dolphins vs. newborns + restores levels of neurotransmitters after day’s activities, explains high level of brain activity & supported by SSRI reducing REM activity (links w/ affect disorders) Horne – distinguishes between core sleep (SWS + REM for body and brain restoration) & optional sleep (made up of the other stages which aren’t necessary but can help restore) this combines evolutionary and restoration