Anyone who has pulled an all-nighter knows it is possible to be tired without being sleepy. The body slows and concentration slips, even as thoughts spin toward a manic blur. It feels as though the sleep-deprived brain is actually becoming more active. And indeed it is, according to a recent study in the journal Cerebral Cortex.

Marcello Massimini, a neurophysiologist at the University of Milan in Italy, found that the brain becomes more sensitive as the day wears on. The experiment, he explains, is like poking a friend in the ribs to see how high he jumps. Massimini prodded brain cells in the frontal cortex with a jolt of electricity, delivered via noninvasive transcranial magnetic stimulation. Then he observed how the rest of the brain responded, comparing results from subjects who had been awake for two, eight, 12 or 32 hours. “I'm sure if you bump your friend when he's sleep-deprived, he's going to jump higher,” he says. The sleep-deprived brain, it turns out, also gets jumpy, responding to the electrical jolt with stronger, more immediate spikes of activity.

The results jibe with a widely held theory that while we are awake, our neurons are constantly forming new synapses, or connections to other neurons, which ramps up the activity in our brain. Many of these connections are irrelevant, but the only way to prune them is by shutting down for a while. The theory explains why it is difficult to cram new information into a sleepy brain. But it also helps to explain some unusual medical observations: epileptics are more likely to have seizures the longer they stay awake, and severely depressed patients with abnormally low brain activity sometimes improve after skipping sleep. “You keep them awake for one night, and, incredibly, they get better,” Massimini says.