Table 3.

Overview of the included studies

Study	Study design	Blinding method	Within- or between-subjects design	Sample size	Abstinence duration (in hours)	Method of administration	Caffeine dosing	Type of control	Results
Redondo et al. [38]	Crossover study	Double-blind	Within-subjects	22	12	Oral (capsule)	4 mg/kg	Baseline measurement and Placebo	Caffeine significantly increased pupil size compared to placebo
Lanini et al. [39]	Randomised, placebo-controlled, independent-group design study	Double-blind	Between-subjects	60	12	Oral (capsule)	Habitual breakfast dose of caffeine (Ranged between 25 and 300 mg)	Baseline measurement and Placebo	Baseline difference in pupil size between groups; no interaction with time
Abokyi et al. [40]	Double-blind, placebo-controlled, randomised crossover experimental design	Double-blind	Within-subjects	50	168	Oral (drink)	250 mg	Baseline measurement and Placebo	Pupil diameter increased from 3.4 (± 0.4 mm) at baseline to 4.5 (± 0.72 mm) at 90 min
Krueger et al. [41]	Placebo-controlled, within-subject design	Simple-blind	Within-subjects	5	24	Oral (drink)	350 mg caffeine/75 kg bodyweight	Baseline measurement and Placebo	No influence of caffeine on pupil dilation caused by pain
Wilhelm et al. [44]	Open study design	No blinding	Within-subjects	20	9	Oral (drink)	Caffeine content of 86.85 mg per cup (about 1–1.5 mg/kg)	Baseline measurement	Caffeine caused a reduction of pupillary unrest index (PUI)
Bardak et al. [42]	Non-randomised design	No blinding	Within-subjects	30	12	Oral (drink)	57 mg	Baseline measurement	tendency of increased pupil size in the caffeine group
Nicholson et al. [43]	Randomised controlled trial (RCT)	Double-blind	Within-subjects	6	32	Oral (ingestion)	300 mg	Baseline measurement and placebo	No significant changes in pupil size