It's final exam season again, and for students across the country, the midnight oil is burning bright. Yet a question looms: It's the morning after an intense all-night cram session, and there's a little bit of time before that big test. Should you take a short nap? Or is it better to just stay up? I decided to ask Dr. Christopher Landrigan, Director of the Sleep and Patient Safety Program at Brigham and Women's Hospital.

For more with Dr. Landrigan on the science of sleep and sleep deprivation, check out the Q&A below.

NOVA: How did you get interested in studying sleep deprivation?

Landrigan: My interest in patient safety and sleep deprivation began in my own training as a resident at Children's Hospital. Fifteen years ago, there were no limits on how much doctors in training could work, and I often found myself at the tail end of a 36 hour shift, not performing my best. I got interested in this later. Was there an impact this might have on the safety of our patients?

NOVA: Why do we feel sleepy?

Landrigan: There are a number of different factors that drive human alertness and performance. The first is something called the sleep homeostat. It's a seesaw system: The longer you've been asleep, the greater the drive to wake up, and the longer you've been awake, the greater the drive to sleep. There's also the circadian system, a 24 hour biological clock that tends to drive maximum performance in the daytime, and drives maximum sleepiness in the early hours of the morning. If we stay up past the point where our bodies are telling us to go to sleep, then the sleep homeostat system and the circadian system both start working together to force you to go to sleep.

NOVA: What happens when you don't sleep?

Landrigan: We know that reaction time goes down, that your ability to put together complex puzzles in your mind, even to do simple math seems to deteriorate. It's not exactly understood how sleepiness contributes to degradation of cognitive function, but we know from imaging studies that the ability of the brain to metabolize glucose, to do what it's supposed to do, tends to decrease after you've been up too many hours in a row.

There have been a number of studies that have tried to look at the effects of sleep deprivation on performance and compare those to the effect that's induced by alcohol. It's been consistently shown that wakefulness of about 17-20 consecutive hours leads to performance decrements that are more or less equivalent to those induced by a BAC [blood alcohol content] of 0.05. And at the 24 hour mark, on average we will perform as if we had a BAC of 0.1, which is beyond the legal limit. With sleep inertia, sometimes the sleep decrements can be even greater than that 24 hour level.

NOVA: What's sleep inertia?

Landrigan: It's a slowness in the brain, an inability to react as quickly or to perform cognitive functions as well in the first few minutes after awakening. Sleep inertia is really the idea that the brain doesn't go from zero to 60 in six seconds. In fact it can take many minutes to even a couple hours for it to wear off.

If somebody is sleep deprived on a regular basis, there's a tendency to go into a deeper stage of sleep more quickly, and when woken from deeper stages of sleep, it appears that sleep inertia is most profound. So if you are sleep deprived and you're rapidly woken from sleep for whatever reason, the tendency to have sleep inertia where performance is really impaired for a long period of time is at its worst.

NOVA: What's so bad about not getting enough sleep?

Landrigan: We're only beginning to understand what the adverse consequences of this may be. From long term studies, it's pretty clear that not only do performance failures increase after sleep depriving ourselves, but there's a really a very substantially increased risk of safety problems--for drivers, for people in high risk occupations in risky industries, as well as long term health consequences of this sort of thing. We know that people who deprive themselves of sleep or work shift work on a regular basis are at higher risk of diabetes, metabolic syndrome, heart disease, hypertension, obesity, and so on for years to come.

To the extent that one can, thinking a little bit constructively about schedules and planning a little bit ahead to minimize all nights and minimize sleep deprivation. It's going to do you a world of good down the road.