Think twice before pulling your next all-nighter.

A lack of sleep may cause severe psychological disturbances, according to a study of 26 university students—more than half of them from Harvard.

In the experiment conducted by Harvard Medical School researchers, some students slept eight hours, while the other half stayed awake for 35 hours.

Near the end of the second day, students were placed in an MRI scanner, as pictures flashed before them, said the study’s leader, psychologist Matthew Walker of University California at Berkeley.

The first were relatively neutral, such as an empty wicker basket. But near the end of the 100-image sequence, the photos became more disturbing, such as one of severed limbs.

Walker said students were initially “very amiable characters” but became increasingly “pendulum-like in their emotions.”

The students proved to be “sort of punch-drunk giddy on sleep deprivation,” he added.

All but one of the Harvard students, Walker said, reported pulling an all-nighter—or regularly doing so.

Paul Barreira, director of behavioral health and academic counseling at University Health Services, said most college students get less sleep than the suggested eight hours.

“If someone is being treated for anxiety and depression,” he said, “sleep deprivation can make it worse even in the face of treatment.”

Last year Harvard freshman participated in a study to observe the sleep cycle, keeping journals about their habits, Barreira added.

Students in the Yard yesterday admitted to erratic sleeping cycles.

Anjali M. Bhatt ’11 said she had gotten only three hours of sleep the night before.

Max A. Newman-Plotnick ’11 boasted yesterday that his “crowning achievement [at Harvard] is a 52-hour period with only 3.5 hours of sleep.”

Walker cautioned against such behavior, saying students would get more work done with their work if they received the full recommended eight hours of sleep a night.

“Remember what your mother told you,” he said. “You need to get a good night’s sleep.”