Chronic non-24-h light–dark cycles alter healthspan measures but not overall survival in male C57BL/6 J mice

Circadian biology is recognized for its importance to physiology and metabolism, with implications for healthy aging and disease risk. To test whether altering the number of imposed environmental cycles experienced per unit chronological time affects health biomarkers and lifespan, we compressed and expanded the photic cycle (T-cycle, combined light:dark length) by ~ 10% relative to the 24-h day. Male C57BL/6 J mice ( n = 60 per group, 12 weeks old) were randomized to one of three T-cycles: T ~ 22-, T = 24-, or T ~ 26-h. Seasonal variations in light:dark proportions of the photoperiod from 8:16 to 16:8 and back over 365 imposed photic cycles were applied to the T = 24 condition and scaled (+ / − 10%) for each. Body weight and food intake were measured weekly under ad libitum feeding, with body composition, metabolic rate, and functional measures collected. Mortality was measured daily, and survival was compared by Cox and linear mixed models (LMM). T-cycle group differences for health biomarkers were assessed with LMM. Mice gained weight early in life across all three T-cycles, with significant effects of time, T-cycle, and time*T-cycle interactions ( p < 0.001, p < 0.001, p < 0.001; T ~ 22 quicker weight gain). Similar results were found for fat mass and lean mass, where significant ( p < 0.05) time, T-cycle, and time*T-cycle interactions were observed. There was no significant difference among T-cycles for overall survival, mean longevity, or maximum lifespan ( p = 0.086, p = 0.079, and p = 0.527, respectively). Mice experiencing 10% chronic contraction or extension of the daily T-cycle do not show significant differences in mortality versus the 24-h T-cycle controls.