Clocking immunity: circadian modulation of NK cells and emerging timing perspectives.

Natural killer (NK) cells show day-night variation in both number and effector function. We compile findings from human cohorts, animal models, and cell studies on how circadian timing shapes NK biology. Evidence spans daily changes in counts and readouts such as degranulation and IFN-γ; links to core clock modules (PER1/2, NFIL3/E4BP4, STRA13); and neuroendocrine inputs (sympathetic tone, glucocorticoids, melatonin). Acute sleep loss can transiently raise NK activity, whereas multi-day sleep deprivation or circadian misalignment lowers counts or function. Shift work studies and laboratory night-shift simulations show reductions in NK activity and phase-sensitive changes in transcriptional programs (e.g., AP-1/STAT), with effects amplified by irregular schedules and accumulated fatigue. Across cancer, depression, vitiligo, and infection, alterations are heterogeneous, often presenting as peak shifts or amplitude flattening rather than loss of rhythmicity. Photoperiod, season, age, and sex can modify these patterns. Current data support a circadian influence on NK biology, but the direction and magnitude of reported effects vary across species, sampling schedules, circadian phase definitions, tissue compartments, and NK-cell readouts. This heterogeneity underscores the need for longitudinal, high-frequency sampling with complementary continuous monitoring to define phase, amplitude, and stability across physiological and clinical contexts.