Do you make a point of going to bed early so that you can get eight solid hours of uninterrupted sleep? Well, YOU'RE DOING IT WRONG. Human sleep cycles were a lot different just a few hundred years ago, with waking up for a few hours in the middle of the night being the norm. We can likely blame technology for the change, and it may not be for the best.
A few decades ago, psychiatrists at the National Institutes of Mental Health performed an experiment where they shut a group of people up in a lab, gave them 14 hours of darkness every night for an entire month, and told them to sleep as much as they wanted, whenever they wanted.
Instead of sleeping more than normal, the volunteers in the study slept about the same: eight or nine hours a night. But by the fourth week, they'd completely changed their sleep patterns: instead of sleeping straight through, they slept for four or five hours, woke up for a few hours, and then slept for another four or five hours. The first sleep period, in the evening, was characterized by deep, slow-wave sleep, while the second sleep period, closer to the morning, was REM sleep full of vivid dreams.
Most interestingly, however, was the wakefulness period in between: instead of freaking out about being awake in the middle of the night, brain measurements showed that most people were calm to the point of being in a meditative state.
So if this is the type of sleep that humans default to when given the opportunity, why aren't we more familiar with it? Turns out, if you look back a few hundred years, having two sleep periods wasn't just familiar, it was the norm. Through the 15th and 16th centuries, historians have found hundreds of references (in diaries, court records, medical books and literature) to "first sleep" and "second sleep," and a contemporary French doctor's manual even suggested that between the two was the very best time to have sex.
The point at which these references to bi-modal sleep cease to appear is just about the same time as electric lighting begins to be introduced throughout Europe. For the first time, people could stay up at night being productive and having fun, which not only cut down on the overall amount of sleep that they got, but also created a perception that lying in bed doing nothing was a waste of time, a perception that was intensified by the industrial revolution and persists to this day.
This isn't to say that getting eight hours of sleep in a row is bad for you, it just may not be the best for you, and that lack of restful, non-anxious night meditation time could potentially have been a way for people to regulate stress that we've since abandoned. So just chill out and relax if you find yourself awake in the middle of the night, and remember that this is how your body is set up to work in the first place.