Performance, Mood, and Brain and Metabolic Functions During Different Sleep Schedules

This proposed project will investigate whether a variable or a stable sleep schedule will be more effective in minimizing neurobehavioural and metabolic deficits when total sleep opportunity across two weeks is below the recommended sleep duration. This project involves young adults aged 21 - 35. For the two weeks leading up to the stay-in study, participants' sleep-wake patterns will be assessed via actigraphy and sleep diary. For the first week, participants will be asked to follow their habitual sleep schedule, while in the following week, the participants will be instructed to follow a specific sleep schedule with an 8-h sleep opportunity each night during the 7-night period with no napping allowed.

After the two weeks of actigraphy, participants will begin the 16-day in-lab protocol. Participants will be randomised into 1 of 3 groups. All the participants will have an 8-h sleep opportunity during the first two nights (baseline nights) that simulate longer sleep opportunities typical of weekends. During the next 5 'weeknights', the participant's sleep opportunities will be manipulated depending on their assigned group. The stable short sleep group will have a 6-h TIB in each of the following 5 'weeknights' (8866666). The variable short sleep group (8884846) will also have a total TIB of 30h during the 'weeknights', although TIB varies across the 'weeknights'. The nightly TIB of the well-rested control group will be 8h (8888888). The same sleep schedules will be implemented in the second week. The protocol will end with an 8-h sleep opportunity on night 15. For each participant, wake times will remain the same (i.e., participant's own habitual wake time derived from self-report and actigraphy) for all the TIBs. Thus, bedtimes will be delayed progressively with decreasing TIBs for the short sleep groups.

A battery of cognitive tests and psychological scales (approximately 30 minutes) will be administered 5 times a day at 3-hourly intervals, starting from 1.5 hours since awakening.

A memory encoding task will be administered on Day 8 to determine whether a variable/stable short sleep schedule is less disruptive to the acquisition of long-term memory. During the task, participants will be presented with 160 images containing a stimulus (e.g. landscapes) or no stimulus on a computer screen. Each image will be displayed for 2500ms followed by a response screen to prompt participants to indicate if a stimulus was previously presented. On Day 10, a randomized set of 240 images consisting of the 160 images previously shown (i.e. "old" images) and 80 "new" images will be presented to participants. Participants are tested on their recognition of the "old images" by selecting on a 5-point scale: (1) definitely did not see, (2) probably did not see), (3) unsure, (4) probably saw, (5) definitely saw. The two tasks will take approximately 25 minutes each.

An OGTT and fMRI brain scan will be performed after the second baseline night 2 (Day 3) and the last 'weeknight' each week (Day 8 and 15). Participants will be asked to perform 8 hours of overnight fasting before each OGTT. On the morning of the OGTT, an intravenous catheter will be inserted into the forearm of participants. Thereafter, 6 mL of blood samples will be drawn in a lying/sitting position. Participants will then be given a 75-g glucose solution to finish drinking within 7-10mins. 6 mL of blood samples will be collected again at time 15, 30, 60, and 120 minutes to measure for changes in glucose and insulin levels.

In addition to OGTT, the investigators will be using continuous glucose monitoring to measure glucose responses to the various sleep schedule throughout the 16-day study via a glucose sensor applied to the back of the participants' upper arms. To ensure the validity of the continuous glucose data, the investigators will provide three main meals each day to the participants, except on the OGTT days when only lunch and dinner will be provided. The portion of the food provided each day will ensure that each participant will consume the daily calorie and macronutrient requirement, and thus, the participant will not gain or lose weight during the study. Participants will be required to finish all the food provided.

During the fMRI brain scans, resting-state and task-related brain activity will be recorded. During part of the scan, participants will be required to perform the gradual onset continuous performance task (Esterman et al., 2013) wherein participants will be presented with grayscale photographs of different scenes (e.g. mountain and city scenes) in a random manner. Each scene is presented for 1600ms with a 800ms overlap with an interpolated transitions. Participants will then be instructed to press a button if a particular scene is identified (e.g. city scene), and withhold their responses to other scenes. Before the task, participants will be given the opportunity to get familiarised with the images and have a 1-minute practice session.

Sleep macro-structure (i.e., the duration of various sleep stages) and micro-structure (such as SWA) will be measured with polysomnography (PSG) every night. Electrodes will also be affixed to the participant's scalp for electroencephalographic (EEG) recording, around the eyes for electrooculographic (EOG) recording, under the chin for electromyographic (EMG) recording, and on the chest for electrocardiogram (ECG) recording. Pulse oximetry, a non-invasive method that employs a light sensor on a finger cuff, is used in the first baseline night for oxygen saturation measurement to verify that the participants do not suffer from sleep apnea.

Throughout the 16-day in-lab protocol, participants will not be allowed to leave the lab premises or engage in any strenuous exercise. When there are no research procedures being carried out, participants are free to spend their spare time on any activity, with the exception of napping, exercising, or activities that requires the participants to leave the lab premises. Participants will be under constant supervision by the research staff.

Inclusion criteria

• 21-35 years of age
• Healthy
• BMI between 18.5 and 24.9
• Not habitual short sleepers
• Not extreme chronotypes
• Not a shift worker
• Not a smoker
• Daily consumption of ≤ 5 cups of caffeinated beverages
• Weekly consumption of ≤ 14 units of alcohol
• Do not intend to travel across > 2 time zones 1 month prior to the experiment
• Not a fussy eater
• Do not have any food allergy
• No strict dietary requirements
• No intention to lose or gain weight in the next 6 months
• Not pregnant during the study
• Do not have any metallic implants
• Do not suffer from claustrophobia (last three inclusion criteria are only applicable for fMRI)

Exclusion criteria

• The reverse of the inclusion criteria