Effect of 60 days of head down tilt bed rest on amplitude and phase of rhythms in physiology and sleep in men

Twenty-four-hour rhythms in physiology and behaviour are shaped by circadian clocks, environmental rhythms, and feedback of behavioural rhythms onto physiology. In space, 24 h signals such as those associated with the light-dark cycle and changes in posture, are weaker, potentially reducing the robustness of rhythms. Head down tilt (HDT) bed rest is commonly used to simulate effects of microgravity but how HDT affects rhythms in physiology has not been extensively investigated. Here we report effects of −6° HDT during a 90-day protocol on 24 h rhythmicity in 20 men. During HDT, amplitude of light, motor activity, and wrist-temperature rhythms were reduced, evening melatonin was elevated, while cortisol was not affected during HDT, but was higher in the morning during recovery when compared to last session of HDT. During recovery from HDT, time in Slow-Wave Sleep increased. EEG activity in alpha and beta frequencies increased during NREM and REM sleep. These results highlight the profound effects of head-down-tilt-bed-rest on 24 h rhythmicity.

Campaign is referred to the leg the study was split into (January-April or September-December). Group refers to the cocktail treatment group (results detailed below in this supplementary material).Segment is the phase of the study: baseline data collection (BDC), head-down tilt (HDT) or recovery (R).Num DF: numerator degrees of freedom; Den DF: denominator degrees of freedom.Campaign is referred to the leg the study was split into (January-April or September-December). Group refers to the cocktail treatment group (results detailed below in this supplementary material).Sampling session refers to the specific day the data were collected: twice during baseline data collection (BDC): BDC1 (-12/-11), BDC2 (-4/-3); three times during head-down tilt bed rest (HDT): HDT1 (1/2), HDT2 (26/27) and HDT3 (53/54); and once in recovery (R): R (+10/+11).Num DF: numerator degrees of freedom; Den DF: denominator degrees of freedom.Supplementary Table 4. Differences of least squares means for segment (BDC, HDT, R) and sampling sessions (BDC1, BDC2, HDT1, HDT2, HDT3, R).The scheduling of campaigns had a significant effect on the onset of melatonin rhythms (Table S5) reflected by an interaction between campaign and sampling session (p = 0.0053).In campaign  Subjective sleepiness at breakfast was significantly higher during all segments of campaign 2 than campaign 1 (p = 0.0057).This could be due to the previously described seasonal effect on sleepiness in the morning.
An important aim of this project was to confirm the effectiveness of an antiinflammatory cocktail as a countermeasure for microgravity, to be used by astronauts to alleviate adverse effects of microgravity.Regarding the effects of this cocktail in our biological rhythms and sleep, it seemed to have an effect only on mean levels of melatonin and cortisol.
Melatonin levels were lower in the group treated with cocktail (3.75 ± 0.23 pg/mL) than in controls (5.24 ± 0.28 pg/mL) (p = 0.0223), but no interaction between group and sampling session was found.Since these differences were already present in BDC before the start of the treatment they probably do not represent an effect of treatment.
Cortisol levels were, in general, lower, in the group treated with cocktail (6.36 ± 0.21 nmol/L) than in control (8.29 ± 0.22 nmol/L) (p = 0.0025), but no interaction between group and sampling session was found.
There was no significant difference in subjective sleepiness between control and cocktail groups when considering the whole study.However, at HDT1 the KSS score

Table 2 .
Differences of least squares means for segment on variables measured across the study.

Table 3 .
Type 3 Tests of Fixed effects for variables measured during specific sampling sessions.Factors: campaign, group, sampling session, and interactions.
concentration for each sampling session and campaign (C1, from January to April; C2, from September to December).