Overview: Virtually every animal sleeps. But why? Why do we need to spend nearly a third of our lives in such a defenseless state?

Scientists are peering more deeply into the sleeping brain than ever before, discovering just how powerful sleep can be, playing a role in everything from memory retention and emotional regulation to removing waste from our brains. So why are we getting so little of it?

DVD Verdict: Yet another fully satisfying, and totally enthralling new NOVA PBS DVDs, 'Mysteries of Sleep' reveals, of course, we all need it, but that we don't always do it well, or as much as we should, or on any sort of regular schedule; oh, and humans do it differently than some other creatures!

Not even all humans do it the same way, yet we still don't really understand its function, or why it's so critically important.

Despite the fact that it's been a universal of human experience for our entire existence as a species, it remains one of science's greatest mysteries.

What is its purpose? Why did it evolve to claim a third of each person's lifetime? What effect does its disruption or loss have on our health and well-being?

We tend of think of sleep as something done only by humans and other mammals: dogs, cats, humans. But although creatures with more primitive nervous systems don't sleep quite the same way as more complex animals, they do in fact display regular sleeping behaviors.

Those behaviors can provide important insights into the entire phenomenon of sleep in all animals.

One of the most exciting biological discoveries of recent years is the revelation of the many natural clocks that govern our lives.

Rather than just a single "master clock" in the brain that controls our circadian rhythms of sleeping and waking, there are many other molecular clocks in the body's organs and even in some individual cell types that govern metabolism and other bodily cycles.

It's revealed that the master sleep-wake cycle is intimately tied into these other rhythms, and when those rhythms fall out of sync the physiological dissonance created can have far-reaching consequences.

Amongst other seriously enthralling revelations, it turns out that losing sleep doesn't just make us feel sluggish, tired, and punchy, it also causes profound changes in the body that can lead to major health problems.

Indeed, it's been shown that sleep deprivation can result in the unfolding and misfolding of proteins in the brain and in the cells of the pancreas, particularly as we grow older and normal repair functions such as the UPR (unfolded protein response) begin to break down.

For more information and insight like this, I urge you to check out the brand new 'Nova: Mysteries of Sleep,' out now on DVD. This is a Widescreen Presentation (1.78:1) enhanced for 16x9 TVs.