Methods

Subjects were 10 healthy young men. Due to problems with blood sampling, data from one man were excluded. The remaining nine subjects’ age was between 20 and 40 years (mean ± SEM: 24.2 ± 1.0 years). The BMI ranged between 20.7 and 25.0 kg m−2 (23.8 ± 0.6 kg m−2). All subjects reported a stable body weight (±1 kg) within the last 3 months and did not follow a particular diet. Only subjects with a regular sleep–wake cycle were included. Shift workers were excluded from participation. All participants were instructed not to deviate from their regular sleep and eating habits during the study. To furthermore avoid any systematic influence of potential alterations in sleep habits during the study period, subjects were examined in randomized, balanced order in the different conditions. Self‐reported habitual sleep time was 7–8 h (mean ± SEM: 459 ± 7 min) with time to bed ranging between 22:00–00:00 hours (23:16 hours ± 5 min) and wake‐up time ranging between 06:00–08:00 hours (07:07 hours ± 10 min). The examinations reported here were part of larger study designed to test effects of sleep on hypoglycaemia counter‐regulation (Schmid et al., 2007). The study protocol was approved by the ethics committee of the University of Lübeck, and all participants gave written informed consent.

Subjects were examined on three different conditions spaced at least 2 weeks apart and performed in a balanced order. Conditions were as follows: (i) a night of total SD, (ii) a night of sleep time restriction to 4.5 h during the first part of the night and (iii) a night with 7 h of sleep.

Participants were instructed not to consume caffeine (for 10 h) and alcohol (for 24 h) on the day before the experiments, and to avoid extraordinarily intense physical activity. Compliance with these instructions was assessed by systematically interviewing the subjects before each experimental night. After arrival at 21:00 hours at the research unit, subjects were allowed only to drink water but to abstain from any intake of food until the end of the experiment. In the 7‐ and 4.5‐h sleep conditions, subjects went to bed following the preparation of polysomnographical recordings. Until lights were turned off at 22:30 hours, they read and talked to the experimenter. Depending on the experimental condition, subjects were awakened either after 5 h or 7.5 h of bed time. In these conditions, subjects slept on average approximately 4.5 and 7.0 h, respectively. In the 4.5‐h sleep condition, participants were not informed before that sleep ended after 4.5 h, so that early awakening took them by surprise. During the rest of the night, subjects read and watched a movie in a sitting position within the laboratory illuminated by standard office lighting (∼500 lx). Brisk physical activities were not allowed. The same setting was used during total SD. Sleep was recorded polysomnographically in both sleep conditions. Sleep recordings were analysed offline according to standard criteria by Rechtschaffen and Kales (Rechtschaffen and Kales, 1968). During SD, subjects were monitored by the experimenter throughout the night.

Blood samples for determination of serum leptin and plasma ghrelin levels were drawn at 07:00 and 07:30 hours. Immediately thereafter, subjects rated their feelings of hunger from 0 (not at all) to 9 (severely) on a standard symptom questionnaire (Schultes et al., 2005). Serum leptin concentrations (Human Leptin RIA KIT; Linco Research, St Charles, MO, USA) and total plasma ghrelin concentrations [Ghrelin (total) RIA KIT] were determined in duplicate by radio‐immunoassay with the following intra‐assay and inter‐assay coefficients of variation (CV) and limit of sensitivity (LOS), respectively: leptin, CV < 8.3 and <6.2%, respectively, and LOS 0.5 ng mL−1; ghrelin, CV < 10.0 and 14.7%, respectively, and LOS 93 pg mL−1. For statistical analysis, anova for repeated measures were performed including the factor ‘sleep’ (for the different sleep conditions) and ‘time’ (for measurements at 7:00 and 7:30 hours). Because there was neither a significant (P < 0.05) ‘time’ main effect nor a significant ‘sleep’ × ‘time’ interaction in any of the analyses, values obtained at 07.00 and 07.30 hours were averaged and thus reported here.