Circadian rhythms in the chemoreflex control of breathing

Mechanisms underlying the circadian rhythm in lung ventilation were investi gated. Ten healthy male subjects were studied for 36 h using a constant rou tine protocol to minimize potentially confounding variables. Laboratory lig ht, humidity, and temperature remained constant, subjects did not sleep, an d their meals and activities were held to a strict schedule. Respiratory ch emoreflex responses were measured every 3 h using an iso-oxic rebreathing t echnique incorporating prior hyperventilation. Subjects exhibited circadian rhythms in oral temperature and respiratory chemoreflex responses, but not in metabolic rate. Basal ventilation [i.e., at subthreshold end-tidal carb on dioxide partial pressure (PETCO2)] did not vary with time of day, but th e ventilatory response to suprathreshold PETCO2 exhibited a rhythm amplitud e of similar to 25%, mediated mainly by circadian variations in the CO2 thr eshold for tidal volume. We conclude that the circadian rhythm in lung vent ilation is not a simple consequence of circadian variations in arousal stat e and metabolic rate. By raising the chemoreflex threshold, the circadian t iming system may increase the propensity for respiratory instability at nig ht.