I read with great interest the article by Calvin et al (July 2013).1 The authors found that people whose sleep was restricted to approximately 5 h per night over an 8-day period consumed on average 822 more calories per day than people who slept approximately 7 h per night.1 This outcome is in-line with previous study findings. For instance, when sleep-deprived for one night, young adults chose greater portion sizes under both fasting and sated conditions than they do after one night of normal sleep.2 However, in contrast to previous findings,3,4 Calvin et al1 did not observe any differences in activity energy expenditure or circulating concentrations of leptin and ghrelin. Based on their findings, the authors conclude that short sleep duration, if habitual, may contribute to the high and rising prevalence of obesity in our modern societies. However, one methodologic issue of their study requires a more detailed discussion. Although the sleep groups did not differ in terms of age, their participants’ ages ranged from 18 to 40 years.1 During aging—also covering the age period between 18 and 40 years—total sleep time, sleep efficiency, percentage of slow-wave sleep, percentage of rapid eye movement sleep, and rapid eye movement sleep latency all significantly decrease.5 With this in mind, it cannot be definitively discounted, especially in light of the small sample sizes (sleep deprivation group, n = 8 vs sleep control group, n = 9), that the participants’ heterogeneous age masked the effect of sleep restriction on activity energy expenditure and circulating concentrations of leptin and ghrelin.