A:

Though it’s been called an “old wives’ tale with no data to support it,” many athletes swear by the two-nights rule. Coaches have suggested that simply believing in it prevents athletes from worrying that (often inevitable) night-before-the-race sleeplessness will impair performance. And there’s some evidence that performance loss related to a night of fitful sleep may, indeed, be mostly psychological.

A 2009 study published in the European Journal of Applied Physiology found that athletes who ran after staying up for 30 hours experienced decreased endurance performance, but that their sleep deprivation had a limited effect on pacing, cardiorespiratory, or thermoregulatory function. Researchers concluded that the lowered performance was likely more psychological than physiological; one sleepless night altered the subjects’ perception of effort, not their physical abilities.

Other studies have found that a lack of sleep does not affect anaerobic performance or weightlifting performance, though it does lengthen reaction time.

This is where it gets interesting. Researchers have found that sleep deprivation affects metabolism, specifically the body’s ability to metabolize glucose and, therefore, to optimally fuel muscles during endurance exercise. As far back as 1989, researchers suggested that a lowered supply of energy resulting from sleep loss may cause that altered perception of effort that leads to decreased endurance performance.

Since then, it’s been demonstrated several times that sleep deprivation has an adverse effect on metabolism. However, most studies examine the effects of sleep loss over an extended period of time, not just one night.

A study published in 2008 in the Journal of Sleep Research examined the effects of a single night of sleep deprivation on two hormones thought to influence the muscles’ ability to use glucose. Researchers found that one entirely sleepless night did adversely affect metabolism compared to getting seven hours of sleep. Interestingly, however, getting only 4.5 hours of sleep did not have any statistically significant effects on the metabolic hormones compared to getting seven hours of sleep.

THE BOTTOM LINE: As long as you’re not chronically sleep deprived, getting less sleep than usual the night before a race doesn’t seem to have any significant consequences on performance. Not sleeping at all, however, may impair your body’s ability to optimally fuel your muscles on race day. The two-nights rule stands, as long as two nights before your event is one of many nights during which you got enough sleep.