Circadian Rhythmicity in Cold-induced Thermogenesis

Inclusion Criteria: * Dutch white Caucasian males or females * Age: 18-35 years * Lean group: BMI ≥ 18 and ≤ 25 kg/m2 * Obese glucose tolerant group: BMI ≥ 30 and ≤ 42 kg/m2 and fasted plasma glucose levels \< 5.5 and/or 2 h after OGTT ≤ 7.8 mM * Obese impaired glucose tolerant group: BMI ≥ 30 and ≤ 42 kg/m2 and fasted plasma glucose levels ≥ 5.5 and/or 2 h after OGTT between 7.8 and 11.1 mM Exclusion Criteria: * Diabetes mellitus (determined on basis of fasting or OGTT defined by ADA criteria (30) * Any other active endocrine disease (thyroid disease, any signs of Cushing's syndrome, adrenal disease and lipid-associated disorders such as familial hypercholesterolemia) * Any chronic renal or hepatic disease * Use of medication known to influence glucose and/or lipid metabolism or brown fat activity (e.g. beta blockers, antidepressants) * Smoking * Abuse of alcohol or other substances * Pregnancy * Participation in an intensive weight-loss program or vigorous exercise program during the last year before the start of the study * Current participation in another research projects that may influence the current research project * Clinically relevant abnormalities in clinical chemistry at screening (to be judged by the study physician)