Meal size and intermeal interval in human subjects in time isolation.

We analyzed the content and number of meals eaten by eight subjects who lived for several weeks in time isolation and whose free-running sleep-wake periods (SWP) lengthened (to an average of more than 33 hours) and were desynchronized from the stable 25 hour rhythm of body temperature. Recently, in an analysis of meal timing, we reported that the long SWPs of free-running desynchrony (FRD) were associated with significantly longer intermeal intervals (IMI) than the shorter SWPs of free-running synchrony (FRS), suggesting that meals and sleep-wake events are timed by the same mechanism. We now report that in both FRS and FRD, subjects ate approximately three meals per SWP and the average size of a meal was similar across conditions. As a result, mean caloric intake decreased by 21 percent per 24 hours and mean satiety ratio increased by 27 percent on the long biological days of FRD. Despite the decrease in caloric intake, body weight did not change significantly in FRD compared to FRS. Decreased food consumption in FRD was not attributable to lowered core body temperature. The similarity of average meal size in the two FR conditions suggested that increased meal size did not drive the longer IMIs of desynchrony, although variability of caloric intake from meal to meal within conditions correlated with postmeal and premeal intervals. The results are interpreted as further support for the concept that in time isolation, the same mechanisms that governs the timing of sleep and waking is important in regulating the timing of meals.