REF-1728 · Boukhris et al., 2026 · Chronobiology International, · Paper · PubMed

After both a 25-min and a 90-min afternoon nap opportunity, every performance measure was at its worst in the first 30 min after waking (sleep inertia, deeper after the 90-min nap); most measures had improved by 60-90 min after waking, and at 90-120 min the 90-min nap beat no nap and the 25-min nap on sleepiness, fatigue, decision reaction time and grip, in a randomised crossover of 17 physically active young adults (abstract only; no effect sizes in the abstract)

Main caveat: One small laboratory study; the abstract gives only P values, so the size of each effect and its uncertainty are unknown.
limited evidenceMeasured on recreational peopleRead from the abstract only

Boukhris and colleagues tested how long the grogginess after a daytime nap (sleep inertia) lasts and whether it depends on nap length. Seventeen physically active young adults each completed three afternoons in random, counterbalanced order: no nap, a 25-min nap opportunity and a 90-min nap opportunity in a laboratory sleep pod.

Night sleep before each session was tracked with an actigraph and nap sleep with a forehead EEG device. From 15:00 they completed four 30-min test batteries spanning the first two hours after the nap: sleepiness, fatigue and readiness to perform, a vigilance task and a Simon (decision) task, handgrip strength and a countermovement jump.

In the first block (0-30 min after the nap) every performance measure was worse than in the later blocks after both nap lengths, and the dip was deeper after the 90-min nap for sleepiness, readiness, handgrip and decision accuracy.

By the third block (60-90 min) most performance measures had improved after both naps (the abstract does not say whether against no nap or against the first block), and in the fourth block (90-120 min) the 90-min nap beat both no nap and the 25-min nap for sleepiness, fatigue, decision reaction time and handgrip.

The authors conclude that sleep inertia needs at least an hour to clear after either nap length, with gains appearing 1-2 h after napping.

Participants

n = 17 physically active adults, 9 M / 8 F, age 23 ± 2 y.
Sport, training volume, habitual napping, chronotype and usual sleep: not in the abstract.

Protocol

Three conditions in randomised, counterbalanced order: no nap (NN), 25-min nap opportunity (N25), 90-min nap opportunity (N90); naps in a sleep pod in the laboratory.
Prior night: tracked with a wrist actigraph (no restriction is described); nap sleep: forehead-mounted EEG device.
Four 30-min test batteries after the nap: T1 0-30 min, T2 30-60 min, T3 60-90 min, T4 90-120 min, starting at 15:00 (so the naps fell before 15:00; their start times are not in the abstract).
Each battery: sleepiness, fatigue, readiness to perform; psychomotor vigilance task; Simon task (reaction time and accuracy); handgrip strength; countermovement jump.
Not in the abstract: the sleep actually obtained in each nap, sleep stages, the prior-night sleep values, the time between sessions, the scales used and any warm-up before the physical tests.

Key Results

ComparisonFinding (abstract)
T1 (0-30 min after the nap) vs T2-T4, both nap lengthsall performance markers impaired at T1 (p < 0.05)
N90 vs N25 at T1deeper sleep inertia after N90 for sleepiness, readiness to perform, handgrip and Simon-task accuracy (p < 0.05)
N25 and N90 at T3 (60-90 min)most performance markers improved (p < 0.05); comparator not stated in the abstract
N90 vs NN and N25 at T4 (90-120 min)larger improvement after N90 in sleepiness, fatigue, Simon-task reaction time and handgrip (p < 0.05)
No means, effect sizes or confidence intervals are given in the abstract.
How the nap conditions compared with no nap at T1 and T2 is not stated in the abstract; the T1 impairment is reported against the later time points.
Countermovement jump: included in "all performance markers" at T1; no separate result for it later is in the abstract.

Practical Application

After a nap, the first 30 min were the worst time to perform: every measure was better in each later 30-min block; most had improved by 60-90 min after waking, and gains over no nap were reported for the 90-min nap at 90-120 min. This held for a 25-min and a 90-min nap opportunity in 17 active young adults after a night of their own sleep; the 90-min nap cost more at first and gave more at 90-120 min.
SYNTH-07 tells the athlete to nap 20-90 min and be up about 30 min before training. Both nap opportunities sit inside that range and both were followed by better performance 1-2 h later, consistent with the length rule; the minutes actually slept are not in the abstract.The guide keeps the 30 min awake and reports these authors' call for at least an hour as their conclusion, not a tested comparison; the reported data fit the 30-min floor: testing ran in four blocks (0-30, 30-60, 60-90 and 90-120 min after waking); every performance measure was worse in the 0-30 min block than in each later block, the 30-60 min block included; most measures had improved by the 60-90 min block (comparator not stated); and how the 30-60 min block compared with no nap is not in the abstract.The authors conclude that at least an hour is needed for sleep inertia to clear. So the abstract supports not performing in the first 30 min after waking, and does not show that 30-60 min awake is worse than not napping. Jump, grip and computer tasks are not an endurance session.
The study does not bear on the guide's short-night trigger: the prior night was the athlete's own, tracked, not shortened.
Inference, not tested here: a long nap (near 90 min) suits a session 1.5-2 h later; with less than an hour between waking and a key effort, the 25-min nap carried the smaller dip.
Inference, not tested here: a warm-up started soon after waking reads a groggy body, so a heavy-feeling warm-up straight after a nap says less about the day than one an hour later.

Limitations

One small laboratory study; the abstract gives only P values, so the size of each effect and its uncertainty are unknown.
"Nap opportunity": how much each participant slept, and in which stages, is not reported in the abstract, so the dose behind each condition is unknown.
Outcomes are short laboratory tasks (jump, grip, reaction time, ratings); no endurance or sport-specific performance.
The prior night was not restricted; the result says nothing about napping after a short night.
Participants' training level, habitual napping and chronotype are not in the abstract.
Written from the abstract and the PubMed record; full text not read.
How this entry was checked and changed
Why this level

a crossover of 17 (the rubric's B+ band for crossovers of 15 or more) held at B: the abstract gives P values only, no means, effect sizes or variability, and not the sleep obtained in each nap; performance tasks, not markers