And yet experience (and everything science can come up with) shows that we do need sleep, and not getting it plays havoc with our mind/body. There's no reason to differentiate humans from other animals in this respect. Search this site for "sleep" and you'll get a lot of information.

Whether it's safe to sleep is a big question. Hunter-gatherers in the rainforest have been said to find it particularly difficult to get a restful night's sleep, and I imagine this was true for many of our ancestors. Modern living has freed us from more dangers than we can count.

As a snorer of fame and notoriety, I often wonder how come my ancestors managed to pass on a gene that attracts so much attention when I'm defenceless. Luckily I wake quickly and my lightning fast reflexes allied with my fearsome fighting skills make me a considerable foe in the morning.

The necessity of sleeping at all is a mystery to me, but manifestly almost all animals do sleep. I believe sharks swim continuously because they naturally sink, and also rely on movement to flow water over their gills (most fish can adjust their buoyancy, and pump water through their gills, while asleep). Apparantly, some birds stand on one leg because half their mind/body can sleep at a time (I may be very gullible on this).

Also, being active 24x7 uses much more energy, so the time spent hunting and foraging is much higher.

It is all a matter of finding an evolutionary niche. If all the predators worked 9 to 5 with occasional overtime, so could the prey. A significant minority of animals are nocturnal because it is safer for them, or their prey is out then, or there is less competition. Consider web spiders: they spin at night to maximise the day period when their web is available, and then sleep until they feel the tug of an impact: a prey/predator timeshare.

At least for humans and many primates, their general bad sight and clumsiness means laying low at night is the best option. To avoid predators, they sleep in places that limit the opportunity of attack, such as caves, ledges, trees and islands, and typically move away from their latrines and food areas first to avoid detection by smell.

Pete and Paul both make good points. This note is just to emphasise that not all animals do in fact sleep at night, and exactly when they do sleep is an evolutionary adaptation in each case. Some have one active period per day and some have two or more. Some are crepuscular, active in the twilight, some for preference in the dark, and some for various periods during the day, depending on lighting, weather, temperature, and so on. Bats for example appear to be generally nocturnal as a defence from birds, particularly predators.

There certainly is a strong argument for the idea that at least a part of the reason for sleep is to keep one out of trouble during one's in active periods. None the less, I cannot accept that that is the operative reason for sleep in general, though it might well be a major reason for sleep patterns.

Oh, and by the way, most animals have a considerable ability to adapt their behaviour according to environmental needs. For instance, city lights, though they are not necesarily good news, don't seem to disrupt behaviour patterns as much as some people have feared, except for some of the more peculiar species such as visible wavelength astronomers.