Persisting and unaltered circadian rhythms of six healthy young men with a night-work shift every 48 hrs and a 2% CO2 atmosphere during a 4-week span.

Six apparently healthy young males (20 +/- 0.5 years of age) lived in a specially designed laboratory for a 1-week span in normal air, followed by 4 weeks in a 2% CO2 atmosphere and thereafter 1 week again in normal air. Room temperature was 24 degrees C. +/- 1 degrees C.; relative hygrometry 75% +/- 5%. With respect to socio-ecologic time clues and cues, the subjects were not isolated. The subjects' social synchronization was altered only by the shift-work schedule (light-on, 07(00); light-off, 22(30) on normal days). Every other day each subject had a 3-h night task, located between 23(00) and 07(00). Once a week, during 48 hrs (Saturday and Sunday) a set of physiologic variables was documented every 4 hrs in order to study their circadian changes: oral temperature, peak expiratory flow, grip strength, arterial blood pressure, tempo, and urinary pH, volume and potassium excretion. As far as rhythms are detectable (cosinor method) the most striking result is that both rhythm acrophases and amplitudes do not show any statistically significant changes when comparing either night-work versus day-work and/or normal air versus air with 2% CO2. Both 3 hrs of night-work every other day and an unusual amount of CO2 do not alter the parameters characterizing the circadian rhythms considered. The absence of desynchronization during night-work could be related to: 1) the speed of rotation in the shift-work; 2) the short duration of night-work; and 3) the youth of the subjects.