The experience of sleep is different for different animals, and different for humans as they age.

• For reasons that are not clear, the amount of REM sleep each day decreases from about 8 hours at birth to 2 hours at 20 years to only about 45 minutes at 70 years of age. - Source [1]
  

• "Interestingly, REM sleep is found only in mammals (and juvenile birds)." - Source [2]

• Adding to the uncertainty about the purposes of REM sleep and dreaming is the fact that deprivation of REM sleep in humans for as much as two weeks has little or no obvious effect on behavior. Such studies have been done by waking volunteers whenever their EEG recordings showed the characteristic signs of REM sleep. Although the subjects in these experiments compensate for the lack of REM sleep by having more of it after the period of deprivation has ended, they suffer no obvious adverse effects. Similarly, patients taking certain antidepressants (MAO inhibitors) have little or no REM sleep, yet show no obvious ill effects, even after months or years of treatment. The apparent innocuousness of REM sleep deprivation contrasts markedly with the effects of total sleep deprivation (see earlier). The implication of these several findings is that we can get along without REM sleep, but need non-REM sleep in order to survive. - Source [3]

• A wide variety of animals have a rest-activity cycle that often (but not always) occurs in a daily (circadian) rhythm. Even among mammals, however, the organization of sleep depends very much on the species in question. As a general rule, predatory animals can indulge, as humans do, in long, uninterrupted periods of sleep that can be nocturnal or diurnal, depending on the time of day when the animal acquires food, mates, cares for its young, and deals with life's other necessities. The survival of animals that are preyed upon, however, depends much more critically on continued vigilance. Such species—as diverse as rabbits and giraffes—sleep during short intervals that usually last no more than a few minutes. Shrews, the smallest mammals, hardly sleep at all.
  An especially remarkable solution to the problem of maintaining vigilance during sleep is shown by dolphins and seals, in whom sleep alternates between the two cerebral hemispheres (see figure). Thus, one hemisphere can exhibit the electroencephalographic signs of wakefulness, while the other shows the characteristics of sleep (see Box C and Figure 28.5). - Source [4]

There are several odd things going on here that leave me with a lot of questions.

For example, what do juvenile birds and mammals have in common that would lead to REM being present in both?

If a person lived long enough, would they cease to experience REM sleep?

If we can so easily do without REM, why do we have it?

If we can't live without the rest of the sleep cycle, what is going on in the other stages of sleep that is crucial to us?

If sleep is a function of our evolution, why did we end up with this particular kind of sleep? We could have evolved to be almost sleepless like shrews or evolved to sleep in one hemisphere at a time. What benefit is worth this vulnerability?

Patty