Background: Human physical performance can fluctuate across the day due to circadian rhythms and diurnal variations in physiological and psychological functions. Factors such as core body temperature, hormonal secretion, neuromuscular activation, sleep–wake patterns, motivation, and perceived exertion may influence exercise capacity at different times of day. Maximum endurance and maximum strength performance are two key components of physical fitness that may respond differently to diurnal variation. Understanding time‑of‑day effects on these performance outcomes is important for optimizing training schedules, testing protocols, and competition preparation. Methods: This study was conducted as a narrative review of experimental and observational studies examining diurnal variation in maximum endurance and maximum strength performance. Relevant literature published between ۲۰۰۰ and ۲۰۲۵ was identified through searches of PubMed, Scopus, and Web of Science databases. Eligible studies included healthy adults, trained individuals, or athletes who performed maximal endurance tests, such as time‑to‑exhaustion, time‑trial, or maximal oxygen uptake assessments, and/or maximal strength tests, such as one‑repetition maximum, maximal voluntary contraction, or peak torque testing, at different times of day. Results: Evidence from the reviewed studies indicates that both endurance and strength performance may vary according to time of day. Maximum strength and power-related outcomes are frequently higher in the late afternoon or early evening, possibly due to increased muscle temperature, improved neuromuscular function, and greater joint mobility. Endurance performance may also improve later in the day in some individuals, although findings are less consistent and may depend on exercise modality, environmental conditions, chronotype, nutritional status, and prior activity. Morning performance may be lower when testing occurs shortly after waking, particularly if sleep duration or warm‑up is insufficient. However, regular training at a specific time of day may reduce or shift diurnal performance differences. Conclusion: Diurnal variation can influence maximum endurance and maximum strength performance, with many studies suggesting superior performance in the late afternoon or evening. Researchers and practitioners should consider time of day when designing performance tests, training programs, and competition strategies. Future research should examine the roles of chronotype, sleep, and habitual training time in modifying diurnal performance responses.
