Although REM sleep occurs in nearly all mammals. Birds, reptiles and amphibians lack it. Even few mammals like the spiny anteater , which has a remarkably large cerebral cortex, does not show any signs of REM sleep. Deprivation of REM sleep in humans for as much as 2 weeks has little or no effect on behavior. Similarly, patients taking certain antidepressants have little or nor REM sleep, yet show no obvious ill effects, even after months or years of treatment.
Human sleep occurs with circadian periodicity.
The normal circadian rhythm is maintained, but slightly loses its usual relationship to the actual time. Removal of SCN in animals abolishes their circadian rhythm. Recent findings suggest that cells in the SCN contain oscillatory molecular mechanisms that control expression of specific gene products, effectively creating a timing mechanism for circadian rhythm. Sleep and wakefulness are now understood to be generated by the activation of specific neural centers. These nuclei have widespread ascending and descending connections to other regions of the brain, which explains their numerous effects.