Professor: We spend approximately one-third of our lives sleeping. Given that the average life expectancy for most of us falls between 73 and 79 years old, we can expect to spend approximately 25 years of our lives sleeping.

Surprisingly, the reason why humans sleep is still mostly a mystery. However, there are a few theories that attempt to explain the function of sleep.

One popular hypothesis argues that sleep is essential in order for us humans to restore the resources we use throughout the day. In the same way that bears hibernate in the winter when resources are scarce, perhaps people sleep at night to reduce the amount of energy they use.

It has recently become fashionable to argue that people need to sleep at least eight hours a night in order to function at their optimal physical and mental capacity. After all, most of you have felt pretty sluggish after a late night with just a few hours of sleep — so this theory makes sense intuitively.

However, there are others who are not convinced by this simplistic explanation of why we sleep, and they have proposed other explanations.

Another popular theory proposes that sleep is an adaptive response to predatory risk. In other words, our great ancestors were more likely to be hunted and killed at night by other animals. So, a desire to sleep in a safe, comfortable area was an evolutionary mechanism that helped shield us from harm.

As I'm sure you are all aware, there are more risks at nighttime since visibility is so low. We haven't adapted our senses in the same way other creatures have in order to survive throughout the night. Accordingly, it makes sense to believe that sleep is a biological response to our environmental conditions.

But just like the theory involving energy conservation, there are some who disagree.