The Influence of Fluid Intake on Daily Biological Rhythm and Mental Performance in Healthy Young Adults

1-week intervention where the intake of drinking water will either be increased (if habitually low) or decreased (if habitually high). The total fluid intake (TFI) prescriptions during the intervention are derived from the mean TFI of habitually high and habitually low drinkers from a sex, age and country matched population. Specifically, habitually high drinkers will be permitted a TFI of 1.3 L/day and habitually low drinkers will be permitted a TFI of 3.5 L/day for men and 3.3 L/day for women. Participants will be instructed to maintain their usual intake of other beverages i.e., tea/coffee to achieve their target TFI. The diurnal saliva cortisol response at rest will be examined at the end of both experimental periods and the saliva cortisol response to an acute psychological stress will be examined at the end of the habitual experimental period only.