The cost of a sleepless night, in terms of energy expended, is equivalent to walking almost two miles, University of Colorado researchers found in a first-of-its-kind study.

The study, published this month in the Journal of Physiology, was the first to quantify how much energy humans expend during sleep.

"We found that people do expend more energy when they are awake in bed than when they are asleep," lead researcher Kenneth Wright, an associate professor at CU, said in a news release.

The findings showed that eight hours of sleep saved roughly 135 calories over eight hours of wakefulness.

"While the amount of energy savings for humans during sleep may seem relatively small, it actually was a little more than we expected," Wright said.

The seven study subjects stayed for three days in a "whole-room calorimeter" -- a 100-square-foot room with a bed, television and small bathroom. They stayed in bed the whole time, first having 16 hours of wakefulness followed by eight hours of sleep, then going through 40 hours of sleep deprivation -- during which they talked, read or watched movies -- followed by eight hours of recovery sleep.

The room allowed scientists to measure the types of energy the subjects are burning -- fat, carbohydrates or protein -- and their oxygen intake to gather a measurable energy usage, said Dr. Robert Eckel, a CU School of Medicine professor and co-investigator in the study.

The researchers stressed that energy expenditure during sleep deprivation is neither a safe or effective strategy for weight loss, but the study may have implications for those with sleep disorders such as insomnia or sleep apnea.

"The results aren't so much full of take-home advice for the public but rather a starting point for other research based on what was found," Eckel said.