Metabolism is increasingly recognized as being highly regulated by anticipatory biological rhythms (circadian rhythms or "biorhythms"), which are driven by molecular feedback loops, and which are approximately 24 hours long ("circa diem"). These circadian rhythms exist both centrally, in the brain, but also in the periphery, and are specific to many tissues depending on their main biological function or functions. Whereas these circadian rhythms have been thoroughly characterized in other organisms, their role in humans remain poorly understood, partly because of the difficulty in studying these rhythms in peripheral tissues. The investigators therefore aim to characterize these rhythms in primarily skeletal muscle and adipose tissue in healthy young volunteers (using the so-called constant routine paradigm), and how these rhythms interact with one another at various genetic and molecular levels. At the same time, the investigators aim to study how an unhealthy vs. healthy diet can alter these circadian rhythms, and how they interact with circadian rhythms in other tissue compartments such as those expressed by blood cells.

Inclusion Criteria:

• Age 18-33 yr
• Healthy (self-reported) and not on medication
• BMI 18-28 kg/m2 (and waist circumference <102 cm), and weight stable (±5% body weight in past 6 months)
• Non-smoker and non-nicotine user
• Regular sleep-wake pattern, with sleep duration of 7-9.25 hrs per night
• Sedentary to moderately active with regular exercise habits the last 2 months
• Regular daily meal pattern with 3 main meals

Exclusion Criteria:

• Major or chronic illness, e.g. diabetes, renal disease or inflammatory bowel disease
• Current or history of endocrine or metabolic disorders
• Psychiatric or neurological disorders (e.g. bipolar disorder, epilepsy)
• Frequent gastrointestinal symptoms
• Chronic medication
• Any sleep disorder (e.g. irregular bedtimes, symptoms of insomnia)
• Any issues with or allergies against the provided food items or utilized anesthesia
• Shift work in the preceding three months or for a long duration
• Time travel over two time zones in the preceding month
• Too much weight gain or weight loss in the preceding 6 months
• Pregnancy

In the crossover subgroup condition, participants will not be briefed about what diet they will receive before the actual onset of the dietary intervention