Have you ever wondered how weird it is that humans dream? After all, when you're asleep and especially when dreaming, you're entirely helpless against enemies or predators. Given the huge liabilities of dreaming, you'd think that natural selection would have eliminated it by now.

And apparently it's not just humans being. While we don't know whether other creatures dream, ALL mammals and most birds experience Rapid Eye Movement (REM), the body's signal that the mind is dreaming. Mammals have been around for 150 million years and have morphed into many shapes and sizes. But in all that time, dreaming has remained constant.

Dreaming is clearly essential to the proper functioning of the human brain, so much so that evolution has deemed it worth risking a violent death. As a measure of the importance of dreaming, consider this: when humans are completely deprived of REM sleep, they go insane within days--hallucinating, delusional and paranoid.

Similarly, selectively depriving humans of REM sleep results in "weight gain, increased irritability, difficulty concentrating and anxiety." In other words, the symptoms usually associated with "lack of sleep" are actually the result of a "lack of dreaming."

When it comes to dreaming, all sleep is not created equal. Humans go in and out of REM while they're sleeping, but the rhythm increases as the sleep continues. The most intense and frequent REM sleep takes place at about 6 hours and continues for the next two hours.

If you wake yourself up before you've achieved those final cycles, you're depriving yourself of REM and thus hobbling your brain's ability to perform well. To get to those final REM cycles, the average adult needs at least 7 to 9 hours of sleep.

Which leads me to all those preachy articles insisting that people ought to wake up early in the morning. 

Have you ever noticed that those article almost never talk about going to bed early, too? It's always  "Tim Cook wakes at 3:45 a.m. Maybe you should, too" and never "Tim Cook goes to bed at 7:15 p.m. Maybe you should, too."  Why? Because the "wake early" crowd wants to have their cake and eat it, too.

The "rise early" advice pretends you can somehow game the system to get some "extra hours while everyone else is asleep. This implies getting less than 7 to 9 hours of sleep; otherwise you're just shifting your time around.

Look, people have different natural rhythms. Some people are natural early birds while others (like me) are natural nite owls. But both early birds and nite owls (and everyone in between) absolutely need those last two hours of REM sleep.

And contrary to popular belief, you can't train yourself to need less sleep

So here's the rule of thumb: regardless of when you go to bed, if you need an alarm clock to get up, you're not getting enough REM sleep. Ideally, you should sleep until your brain says it's time to wake, which will be AFTER you've gotten your full complement of REM.

Everyone needs at least 7 to 9 hours of sleep, regardless of when they wake up. All the "go getter" willpower in the world does not and cannot override 150 million years of evolution. It's absurd and presumptuous to pretend otherwise.