Children's Sleep Needs

clipped from www.aafp.org Change in Hours of Daytime and Nighttime Sleep with Increasing Age FIGURE 1. In newborns, the amount of sleep is divided fairly equally between night and day. In the normal infant, nighttime sleep gradually becomes consolidated over the first year into a single uninterrupted block of time, and daytime sleep gradually decreases over the first three years. Adapted with permission from Ferber R. Solve your child's sleep problems. New York: Simon and Schuster, 1985.

Shortening Sleep Will Prolong Conscious Life

Reposted from: http://edge.org/q2007/q07_15.html
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MARCEL KINSBOURNE
Psychologist, The New School; Coauthor, Children’s Learning and Attention Problems

Shortening Sleep Will Prolong Conscious Life

Our life span is extending, but the extended life is a dwindling asset. Who would not prefer to live longer while at their peak? The time we spend asleep contributes little to our lifetime of experience, thought and action. Dreaming doesn't seem to add much. Some brain lesions and monoamine oxidase (MAO) inhibitor medications even completely abolish dreams without making any apparent difference. Could we reduce the duration of sleep (both REM and nonREM) while maintaining its benefits for the brain, whatever they might be? I propose that we do need to sleep, but not as long as we do. The duration of sleep may be an outdated adaptation to prehistoric ecological constraints that no longer exist.

Virtually all vertebrates sleep (and invertebrates at least have quiet time). However, the duration of sleep varies wildly across species, from less than 1 hour to 18+ hours a day. For instance, rodents sleep between 8 and 17 hours, primates between 7 and 18 hours. Elephants and giraffes sleep 3-5 hours, squirrels 16-17 and bats 20 hours. The newborn of most species sleep more of each day than the adults, except that newborn whales and dolphins don't sleep at all. Within a species, the inter-individual variation of adaptively valuable traits is thought to be quite limited. Yet some people, in some families, habitually sleep only 2-4 hours a night, and function well for longer each day. Perhaps constraining the duration of sleep is not an adaptive priority in humans.

Three categorically distinct roles for sleep are: (1) maintaining the neuronal circuitry, (2) fostering learning, (3) keeping the organism out of trouble.

(1) Given its ubiquity among vertebrates and other phyla, any neurometabolic benefit of sleep must be very general and basic. If the needs of the brain determine the duration of sleep, its duration should vary systematically with some fundamental neurological variable, such as absolute or relative size of the brain, its energy utilization, the sophistication of behavioral control, or the need to replenish some key neurotransmitter. No such co-variation appears to exist.

(2) The presumed role of sleep in learning is based on continuing rehearsal. Rather than being an adaptation, the learning benefit may be a fortuitous result of the brain's continuing activity during sleep, while it is receiving no fresh information. Since the neuronal show must go on, recently acquired patterns of firing gain priority and are "rehearsed". Whether the memories are useful or useless, they are automatically rehearsed. In any case, the suggested benefit of sleep for human learning cannot be generalized to species that make a living without learning anything much, and yet require sleep.

(3) The substantial differences between people and the enormous difference between species in how long they typically sleep suggest that sleep also serves a species-specific ecological function. This is sleep's other role; sleep conserves energy and keeps animals out of trouble. It takes the members of each species a minimum time per day to make a living, that is, secure their personal survival and take advantage of any reproductive opportunity. This challenge is met anew every day. On this view, how much of the day is needed to meet adaptive goals determines the duration of the default option of sleep.

Continued activity when the day's housekeeping is done would prolong the animal's exposure to the hazards that lurk in the environment, without contributing further to basic survival and reproductive needs. Many species cannot do anything useful in the dark (and some not in the light). They gain nothing from expending more than basal metabolic energy at that time. The genetic imperative to sleep during a predetermined time of day and for a predetermined duration (or even hibernate), takes care of all that. Thus extended sleep time would be a function of the interaction between the individual and its ecology.

Predators need time for hunting; how much depends on attributes of the predator and the prey, such as speed, strength and population density. Herbivore prey needs a minimum time to graze, depending on the animal's bulk and the accessibility of food. How the remains of the day are spent would depend on how readily a secure haven can be found.

Nature is notoriously conservative, and it conserves the genetically driven imperative to sleep. The imperative to sleep is subjectively experienced as antecedent sleepiness, and the fatigue and dysphoric feeling after too little sleep. My thesis is that these feelings do not arise unavoidably from the economy of the brain, but are genetically imposed adaptations. Should a species' ecology undergo radical change, and making a living become sharply easier or more difficult, natural selection will in time reshape sleep duration accordingly.

However, human culture evolves too quickly. Since artificial lighting was introduced, the dark no longer constrains what people can do. Since human activities are oriented as much to future as to immediate goals, all hours of the day have become potentially useful. Further, we have more effective means to secure ourselves than curling up in a quiet place and sleeping. So if a sizeable portion of the adaptation to sleep has the role of a security saving placeholder, then it would be safe to relax that portion of the sleep constraint.

The dictatorial "sleep genes", when identified, need to be modified to require a shorter sleep duration, and the circadian clock genes need to be reset. Will the state of genetic engineering become sufficiently advanced to make this prospect, though less than a sure thing, more than a pipe dream? The good news comes with the fruit fly's sleep, which is uncannily like ours; a mutation in a gene called Shaker reduces the fly's natural sleep duration by two-thirds, from about 12 to about 4 hours within 24, without detriment to the fly's well-being. The bad news is that these mutated flies don't live long. Nonetheless, I am optimistic.

Why Do We Need to Sleep?

Reposted from: http://edge.org/q2007/q07_14.html
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STEVEN STROGATZ
Applied mathematician, Cornell University; Author, Sync

Why Do We Need to Sleep?

In the coming year, we're going to witness a breakthrough in our understanding of what sleep is for.

It surprises most people to learn that this is even a question. Every other basic bodily function—like eating, drinking, breathing, urinating, or defecating—has a pretty clear purpose. Our bodies and brains need food, water, and oxygen to stay alive and to replenish themselves, and if they didn't rid themselves of the byproducts of this metabolism, we'd be awash in our own toxic waste. Likewise, sleep must be for something important. We all spend decades in this strange state, immobilized, unconscious, and vulnerable. But what exactly does sleep do for us?

Parents tell their children they need to sleep because they're tired and need to rest. But of course rest is not good enough. Lying still for eight hours is no substitute for sleep. My own mother had a different theory. She said I needed to sleep because I had too much "sleepy gas." It had been building up all day long, and so I needed to sleep to get rid of it. In fact, scientists observed a long time ago that if you keep a sheep awake continuously for several days and then inject some of its cerebrospinal fluid into another, well-rested sheep, that sheep will fall right asleep, presumably because some naturally-occurring sleep substance had reached a soporific level in the donor. But this line of research never quite solved the puzzle. Although a number of putative sleep substances have now been identified, we're not sure how they might work biochemically, or how sleep (as opposed to mere rest) might break them down.

Other sleep-deprivation studies done in the early 1980's took a more brutal approach, keeping rats awake for weeks until they died from a lack of sleep, and then looking for the precise cause of death. Such studies (now outlawed) could not pinpoint any specific culprits, such as particular organ failures. One striking observation, however, was that the rats ate much more than normal and yet wasted away. Their metabolism seemed to be wrecked. So maybe sleep is for energy regulation, in some unspecified way. Other popular theories are that sleep is for tissue repair, or immune function, or for consolidating learning and memory.

The new development, and the cause for optimism, is an original approach to the problem that makes the first quantitative, testable predictions about the function of sleep. Two physicists, Van Savage (Harvard Medical School) and Geoff West (Santa Fe Institute), have analyzed how sleep varies across mammals of different species. Normally physiological time ticks slower for bigger animals. For example, elephants live much longer than mice and their hearts beat much slower. The interesting thing is that both animals' lifetimes and pulse times scale in the same way with their body mass—in direct proportion to their mass raised to the 1/4 power—with the curious implication that the hearts of mice and elephants will typically beat the same number of times in their lifetime.

What is so strange about sleep in this regard is that it behaves differently from all other physiological times. It's backward. Mice sleep longer than elephants, not shorter—about fourteen hours a day compared to four. Savage and West interpret this as evidence that sleep is related to cellular repair. They note that cells produce destructive byproducts, such as free radicals, in the course of normal metabolism, and they hypothesize that sleep somehow helps repair the damage that ensues. (In this view, the mouse needs to sleep longer to clean up all the byproducts generated by its revved-up metabolism.). Then, using classic laws about how metabolic rate varies across different species, they derive mathematical predictions about how sleep duration should vary with an animal's size. But which size is most relevant—brain size or body size? The key is that they are not proportional. If sleep is for repairing the brain, Savage and West derive one prediction; if it's for repairing the body, they derive a different prediction. When they finally confront their model with the available data, they infer that sleep is mainly for repairing the brain, not the body. So much for beauty sleep.