I often get asked about this.
Sleep hacking, a.k.a. polyphasic sleep or the uberman sleep schedule, is, according to Wikipedia, "a term used to describe several alternative sleep patterns intended to reduce sleep time to two to five hours daily. This is achieved by spreading out sleep into short naps of around 15-30 minutes throughout the day."
Sleep hackers hope to get the most needed stage of sleep and skip the rest. In a nutshell, they hope to sleep less in a day and do more.
Does this work? Yes and no.
Normal sleep occurs in cyclic patterns. People enter sleep in stage 1, then move to 2, 3, 4, back to 2, and then on into REM sleep. To get to the deep good stuff, you need to go through the entire cycle.
When sleep hacking, you don't get a chance to get there by the normal route. Instead, you can create a "sleep pressure" or homeostatic drive for sleep. In essence, you train your body to go right to Slow Wave Sleep or REM.
This in itself isn't a bad thing. Sleep doctors often use this as a technique called "sleep restriction" for people with insomnia. We ask them to stay up late and get up early to build this pressure to help them sleep.
However, another force that sleep hackers must outwit is the circadian pacemaker. The body, based on certain cues like light and exercise, knows when to do things like eat or sleep. By napping every 2.5 hours or so, you are going against this system, which can confuse your body's instincts.
On top of that, you're awake when everyone else is asleep, which can be isolating and depressing. And by napping every two to three hours, it's tough to leave the house and do things that take a big chunk of time -- like going to work, vacationing, finishing a book, or driving home from a long movie.
Many people who've tried sleep hacking have complained about mood changes and the inability to get things done -- which undermines the original reason for trying this experiment in sleep deprivation in the first place.
At the end of the day, while sleep hacking may work for the short term or in a crisis situation (such as piloting a solo flight around the world), for most of us, getting the regular amount of sleep and obeying our natural internal clocks is a happier, healthier, saner, and easier way to get things done.
Basically: You just can't fool Mother Nature or The Sandman.