Importance of sleep

Nobody can sleep with so great tast as cats, I think. It's incredible how they use every possibility for good relax. Seems they forget everything in this moment.

I watch our cats. They have more difficult life. They live not only in the daytime but in the night too.

So, I wanted to know how much animals sleep and ...
Domestic cat 12,1 h
Tiger 15.8 hr
Lion 13.5 hr
Cheetah 12.1 hr
Jaguar 10.8 hr
Dog 10.6 hr
Pig 7.8 hr
Cow 3.9 hr
Sheep 3.8 hr
Horse 2.9 hr
Giraffe 1.9 hr
(Neuroscience for kids -animal sleep)

I thought, who sleeps much, lives not too long, but I found this:
"On the contrary, long-lived mammals may have some characteristic that enables them to get by with less sleep"

Other interesting thing is that some mammals sleep with open eyes, some sleep while swimming, some sleep part time under water..t

Why mammals sleep?
Seems nobody knows it. We are so wulnerable when we sleep. One of explanations of it is this:
"From a perspective of energy conservation, one function of sleep is to replenish brain glycogen levels, which fall during the waking hours."
"Whatever the reasons for sleeping, in mammals sleep is evidently necessary for survival."
"In humans, lack of sleep leads to impaired memory and reduced cognitive abilities, and, if the deprivation persists, mood swings and even hallucinations. The longest documented period of voluntary sleeplessness is 264 hours (approximately 11 days)"

There are many theories and no one is sure his theory is truth.

I think, it is more simple. If your bow is always strained, it will go bad. We are not able to live always strained. We need relax to survive. This is the reason.