Leptin levels were stable across the daytime period under both sleep conditions, which was consistent with the fact that calories were exclusively delivered in the form of a constant glucose infusion. Average total sleep time was 9 hours and 8 minutes when the men spent 10 hours in bed and 3 hours and 53 minutes when the men spent 4 hours in bed (P < 0.01). When spending 4 hours in bed, the participants had mean leptin levels that were 18% lower (2.1 ng/mL vs. 2.6 ng/mL; P = 0.04) (Figure 1, part A) and mean ghrelin levels that were 28% higher (3.3 ng/mL vs. 2.6 ng/mL; P = 0.04) (Figure 1, part B) than when the participants spent 10 hours in bed. The ratio of the concentrations of orexigenic ghrelin to anorexigenic leptin increased by 71% (CI, 7% to 135%) with 4 hours in bed compared with 10 hours in bed. Sleep restriction relative to sleep extension was associated with a 24% increase in hunger ratings on the 10-cm visual analogue scale (P < 0.01) and a 23% increase in appetite ratings for all food categories combined (P = 0.01) (Figure 1, parts C and D, and Table 1). The increase in appetite tended to be greatest for calorie-dense foods with high carbohydrate content (sweets, salty foods, and starchy foods: increase, 33% to 45%; P = 0.06) (Table 1). The increase in appetite for fruits and vegetables was less consistent and of lesser magnitude (increase, 17% to 21%) (Table 1). Appetite for protein-rich nutrients (meat, poultry, fish, eggs, and dairy foods) was not significantly affected by sleep duration (Table 1). When we considered the changes in ghrelin and leptin in an integrated fashion by calculating the ghrelin-to-leptin ratio, the increase in hunger was proportional to the increase in ghrelin-to-leptin ratio (r = 0.87) (Figure 2). Almost 70% of the variance in increased hunger could be accounted for by the increase in the ghrelin-to-leptin ratio.