The efficacy of caffeine in restoring cognitive declines across 24 hours of sustained wakefulness

The study will run for a duration of five weeks and includes a one-week baseline monitoring period and a four-week intervention period. This study will incorporate a randomised, double-blind, placebo-controlled crossover experimental design to assess the effect of three different caffeine doses (100 mg, 200 mg and 400 mg). The intervention is the administration of caffeine to be taken orally in capsule form. Each participant will be exposed to four conditions (including the control) using a Latin square design. Each condition will be separated by a six-day washout period. Interventions include: - 100 mg caffeine administered as 2 x 50 mg doses at 15 and 19 h post wake - 200 mg caffeine administered as 2 x 100 mg doses at 15 and 19 h post wake - 400 mg caffeine administered as 2 x 200 mg doses at 15 and 19 h post wake The intervention will be administered by a member of the research team during each trial and participants will be monitored during the consumption of the capsule. Additionally, saliva samples will be taken in 2 h intervals across each trial to assess salivary caffeine concentrations. Participants will arrive for each overnight trial 9 h after their morning waking with the trial will conclude after 24 h of sustained wakefulness. As such, each overnight trial will run for 15.5 h. Upon conclusion of the trial, participants will be transported home to undertake a subsequent recovery sleep bout. This recovery sleep bout will be recorded using in-home sleep monitoring that includes the Somfit device. Participants will apply the Somfit device to their forehead via an adhesive electrode patch. Subjective sleep measures will be recorded via an electronic sleep diary that participants will complete on their phone via an email link.