After four hours of sleep for six nights, participants' performance was just as bad as those who had not slept for twenty-four hours straight--that is, a 400 percent increase in the number of microsleeps. By day 11 on this diet of four hours of sleep a night, participants' performance had degraded even further, matching that of someone who had pulled two back-to-back all-nighters, going without sleep for forty-eight hours.

Why We Sleep by Matthew Walker