Can consuming caffeine after a nap improve productivity, alertness and safety?

INTERVENTION: Participants will undergo two 37h in laboratory sleep deprivation protocols. Participants will enter the laboratory at 11am on day 1; stay awake all night; and then sleep between 12pm and 6pm on day 2. Participants will remain in the lab until 12am. Participants will undergo 2 conditions, with the order of conditions randomised: 1) 30min nap with 200mg caffeine gum on waking; and 2) a 30min nap with placebo gum. The 30min nap will be allowed between 2 to 2:30am. They will visit the laboratory twice, a week apart, in a within groups, repeated measures design. Each person will be their own control. A 30 minute cognitive performance test battery, consisting of a 10 minute Psychomotor Vigilance Task, Karolinska Sleepiness Scale, Positive and Negative Affect Schedule, and mood Visual Analogue Scales will be administered at 8:30pm and 11pm on day 1 and at 1am, 5am, 9am, and 8:30pm on day 2. A 20 minute learning reversal decision task will be performed at 9pm on day 1 and 7:30am and 8:30pm on day 2. This computerised task is based on a go/no go paradigm. Initially four two‐digit numbers are assigned to the go (response) set and four two‐digit numbers are assigned to the no go (no response) set. Participants have a 750ms window to either respond or withhold a response to the number displayed on the screen. A hypothetical money reward/punishment is displayed to allow participants to determine which numbers are in the go set and which are in the no go set. After a number of trials (56, 60, or 64) the response sets are reversed for another 40 trials. A 40 minute driving simulation task (Yorke Driving Simulator) will be conducted at 7:30pm on day 1 and 6:30am, 10am, and 7:30pm on day 2. A sleep inertia test battery will be conducted post nap starting at 2:30am. Sleep inertia testing will consist of a 3 minute Psychomotor Vigilance Task, Karolinska Sleepiness Scale, and mood Visual Analogue Scales, and performed every 10 minutes (4 sleep inertia tests to map performance in the hour post nap). Venous blood sampling will be at 12:30pm on day 1 and 3:15am, 4:10am, 5:40am, and 11am on day 2 to quantify plasma caffeine concentrations for pharmacokinetic assessment, the impact of the caffeine gum on cortisol as a marker of stress and the impact of the caffeine gum on melatonin sectretion. Hourly saliva sampling will used to measure melatonin levels during wake periods. Data will be analysed using mixed models and pharmacokinetic/pharmacodynamics modelling will be used to investigate the relationship between blood (plasma) caffeine concentrations and various indices of sleep inertia. CONDITION: Shiftwork Sleep deprivation Sleep inertia PRIMARY OUTCOME: 3 minute Psychomotor Vigilance Task Shift in melatonin onset as measured in saliva using radioimmunoassay SECONDARY OUTCOME: 40 Minute driving simulator task using the Yorke Driving Simulator A 20 minute learning reversal decision task (Go/No Go) Change in melatonin secretion profile measured in plasma using radioimmunoassay Neurobehavioural Test Battery (10 minute Psychomotor Vigilance Task, Karolinska Sleepiness Scale, Positive and Negative Affect Scale, Visual Analogue Scales of mood) INCLUSION CRITERIA: BMI between 18.5‐29 kg/m2 Habitual bedtime between 2100h and 2400h Habitual wake time between 0600h and 0900h Less than 2 hours of structured high impact activity per week Currently taking a contraceptive pill or experiencing regular menstrual cycles (females) Valid driver’s licence or sufficient driving experience