The study will test whether the consumption of a low versus a high glycemic index pre-bed drink influences sleep architecture, memory consolidation, nocturnal glucose metabolism, and mood.

Few studies have examined the impact of meal composition on sleep. Most research has focused on carbohydrate intake, although reports are sporadic and confounded by the manipulation of other macronutrients. Nonetheless, the pattern of results appears to depend on the nature of the sleep variable under investigation e.g., latency, duration, or efficiency. The study will determine whether manipulating the glycemic index of a pre-bed drink influences sleep architecture, procedural and declarative memory consolidation, nocturnal glucose metabolism, and mood.

Participants will stay in the sleep laboratory for three nights, each night separated by one week (visit 1, 2, and 3). The first night is an acclimatisation night. The same procedure will be followed on all three nights, as described below.

On the morning of each visit, a continuous glucose monitor will be set up and worn until 8pm the following evening. Participants will be asked to return to the sleep laboratory at 7pm, having fasted for five hours, and asked to consume a standardised evening meal. Several mood questionnaires will be completed throughout the evening and a polysomnography will be set up. Two memory tasks will be administered at approximately 10pm. Immediately after this, participants will consume either water (night 1), or a low glycemic index or high glycemic index drink (night 2 or 3). Participants will retire to bed at approximately 11pm and woken up between 6.30am - 8am, depending on personal preference.

In the morning, the memory tasks will be administered again, as well as two mood questionnaires. Participants will be asked to wear a Pro-Diary watch throughout the day, which measures mood and hunger. At 8pm, both the continuous glucose monitor and Pro-diary watch will be collected from the participant.

Inclusion Criteria:

• Male (given that the menstrual cycle may influence sleep polysomnography)
• Good health
• Not taking any medication known to effect sleep or metabolism
• Normal sleep duration (i.e. between 6.5 - 9 hours of sleep per night)
• A sleep efficiency score above 80% (Pittsburg Sleep Quality Index)
• Normal sleep schedule (i.e. fall asleep between 10pm and 12pm)
• No difficulty sleeping in new places

Exclusion Criteria:

• Diagnosis of any physical or mental disorder e.g. diabetes, cardiovascular disorders, gastrointestinal disorders, sleep problems, epilepsy, or mood disorders
• Unhealthy weight (BMI over 30)
• Drink more than 300mg of caffeine per day (3 - 4 standard sized 8-oz cups)
• Work nights or shifts
• Smoke/vape
• Regularly drink alcohol (>3 glasses per day)
• Used recreational drugs within the last 6 months
• Take part in >3 hours of vigorous exercise per week
• Any known food allergies or intolerances
• Nocturnal eating or regular consumption of alcohol before bed (assessed by the Meal Pattern Questionnaire)

To ensure blinding of participants and researchers, a randomisation list will be produced using Statistical Package for the Social Sciences (SPSS) software by an employee of Swansea University, who is not actively involved in the study.

Blinding will be maintained by the use of encoded beverages to ensure participants, researchers, and the data analysers are unaware of the intervention. The blind will be broken only after statistical analysis of the data is complete.