apeiron said:
Horses also have deep slow wave sleep sleep, but can lock their knees to sleep standing.
Dreaming sleep (REM) is actually shallow in the sense of being near waking, while deep in that there is a tonic inhibition - basically the brain is cut off from musles - so you don't run around acting out your dreams.
I've seen it said, horses need to lie down in REM, but not sure of the truth there.
Horses of course sleep less because of predator issue. All species tune the amount of sleep to the nature of their lives. So cats and bats sleep a lot, horse and deer only a few hours.
Even fish and insects have quiescent periods.
If this is really a topic that interests you, Jim Horne of Loughborough wrote the best evolutionary stuff on sleep.
Hi Iron
Thank you very much for the detailed reply. I would summarize some of the points on sleep topic for my own reference and for others like me:
Sleep is a naturally recurring state of relatively suspended sensory and motor activity, characterized by total or partial unconsciousness and the inactivity of nearly all voluntary muscles. It is distinguished from quiet wakefulness by a decreased ability to react to stimuli, and it is more easily reversible than hibernation or coma. Sleep is a heightened anabolic state, accentuating the growth and rejuvenation of the immune, nervous, skeletal and muscular systems. It is observed in all mammals, all birds, and many reptiles, amphibians, and fish.
In mammals and birds, sleeping is divided into two broad types: rapid eye movement (REM) and non-rapid eye movement (NREM or non-REM) sleep. Each type has a distinct set of associated physiological, neurological, and psychological features. NREM is further divided into three stages: N1, N2, and N3, the last of which is also called delta sleep or slow-wave sleep (SWS).
Sleep proceeds in cycles of REM and NREM, the order normally being N1 → N2 → N3 → N2 → REM. There is a greater amount of deep sleep (stage N3) early in the night, while the proportion of REM sleep increases later in the night and just before natural awakening.
Sleep stages and other characteristics of sleep are commonly assessed by polysomnography in a specialized sleep laboratory. Measurements taken include EEG of brain waves, electrooculography (EOG) of eye movements, and electromyography (EMG) of skeletal muscle activity. In humans, each sleep cycle lasts from 90 to 110 minutes on average, and each stage may have a distinct physiological function.
There is relatively little dreaming in NREM. Rapid eye movement sleep, or REM sleep, accounts for 20%–25% of total sleep time in most human adults and most of the dreaming happens in this stage.
Sleep timing is controlled by the circadian clock, sleep-wake homeostasis, and in humans, within certain bounds, willed behavior. The circadian clock—an inner timekeeping, temperature-fluctuating, enzyme-controlling device—works in tandem with adenosine, a neurotransmitter that inhibits many of the bodily processes associated with wakefulness. Adenosine is created over the course of the day; high levels of adenosine lead to sleepiness. In diurnal animals, sleepiness occurs as the circadian element causes the release of the hormone melatonin and a gradual decrease in core body temperature. The timing is affected by one's chronotype. It is the circadian rhythm that determines the ideal timing of a correctly structured and restorative sleep episode.
Homeostatic sleep propensity (the need for sleep as a function of the amount of time elapsed since the last adequate sleep episode) must be balanced against the circadian element for satisfactory sleep. Along with corresponding messages from the circadian clock, this tells the body it needs to sleep. Sleep offset (awakening) is primarily determined by circadian rhythm. A person who regularly awakens at an early hour will generally not be able to sleep much later than his or her normal waking time, even if moderately sleep-deprived.
Optimal amount in humans:
Newborn - up to 18 hours
Adolescents - 9–10 hours
Adults, including elderly - 7–8(+) hours
Human biological sleep:
http://upload.wikimedia.org/wikipedia/commons/5/5f/Biological_clock_human.PNG
[SOURCE: Wikipedia]
Iron, what I conclude is that most animals and birds can divide their sleeping time into many mini episodes as compared to humans. They don't need to take all their sleep at one time. And during those mini episodes they, probably, shuffle between different stages of sleep. Does the drawn conclusion carry any legitimacy? Please let me know.
Best wishes
Jack