Sleep Is Compromised in -12º Head Down Tilt Position

Recent studies are elucidating the interrelation between sleep, cranial perfusion, and
cerebrospinal fluid (CSF) circulation. Head down tilt (HDT) as a simulation of microgravity
reduces cranial perfusion. Therefore, our aim was to assess whether HDT is affecting
sleep (clinicaltrials.gov; identifier NCT 02976168). 11 male subjects were recruited
for a cross-over designed study. Each subject participated in two campaigns each
comprising 3 days and 2 nights. Intervention started on the second campaign day
and consisted of maintenance of horizontal position or −12◦ HDT for 21 h. Ultrasound
measurements were performed before, at the beginning and the end of intervention.
Polysomnographic measurements were assessed in the second night which was either
spent in horizontal posture or at −12◦ HDT. Endpoints were sleep efficiency, sleep onset
latency, number of sleep state changes and arousals, percentages of N3, REM, light
sleep stages and subjective sleep parameters. N3 and REM sleep reduced by 25.6 and
19.1 min, respectively (P = 0.002, g = -0.898; P = 0.035, g = -0.634) during −12◦
HDT. Light sleep (N1/2) increased by 33.0 min at −12◦ HDT (P = 0.002, g = 1.078).
On a scale from 1 to 9 subjective sleep quality deteriorated by 1.3 points during −12◦
HDT (P = 0.047, g = -0.968). Ultrasonic measurement of the venous system showed a
significant increase of the minimum (P = 0.009, P < 0.001) and maximum (P = 0.004,
P = 0.002) cross-sectional area of the internal jugular vein at −12◦ HDT. The minimum
cross-sectional area of the external jugular vein differed significantly between conditions
over time (P = 0.001) whereas frontal skin tissue thickness was not significantly different
between conditions (P = 0.077, P = 0.811). Data suggests venous congestion at −12◦
HDT. Since subjects felt comfortable with lying in −12◦ HDT under our experimental
conditions, this posture only moderately deteriorates sleep. Obviously, the human body
can almost compensate the several fold effects of gravity in HDT posture like an affected
CSF circulation, airway obstruction, unusual patterns of propioception and effects on the
cardiovascular system.

Recent studies are elucidating the interrelation between sleep, cranial perfusion, and
cerebrospinal fluid (CSF) circulation. Head down tilt (HDT) as a simulation of microgravity
reduces cranial perfusion. Therefore, our aim was to assess whether HDT is affecting
sleep (clinicaltrials.gov; identifier NCT 02976168). 11 male subjects were recruited
for a cross-over designed study. Each subject participated in two campaigns each
comprising 3 days and 2 nights. Intervention started on the second campaign day
and consisted of maintenance of horizontal position or −12◦ HDT for 21 h. Ultrasound
measurements were performed before, at the beginning and the end of intervention.
Polysomnographic measurements were assessed in the second night which was either
spent in horizontal posture or at −12◦ HDT. Endpoints were sleep efficiency, sleep onset
latency, number of sleep state changes and arousals, percentages of N3, REM, light
sleep stages and subjective sleep parameters. N3 and REM sleep reduced by 25.6 and
19.1 min, respectively (P = 0.002, g = -0.898; P = 0.035, g = -0.634) during −12◦
HDT. Light sleep (N1/2) increased by 33.0 min at −12◦ HDT (P = 0.002, g = 1.078).
On a scale from 1 to 9 subjective sleep quality deteriorated by 1.3 points during −12◦
HDT (P = 0.047, g = -0.968). Ultrasonic measurement of the venous system showed a
significant increase of the minimum (P = 0.009, P < 0.001) and maximum (P = 0.004,
P = 0.002) cross-sectional area of the internal jugular vein at −12◦ HDT. The minimum
cross-sectional area of the external jugular vein differed significantly between conditions
over time (P = 0.001) whereas frontal skin tissue thickness was not significantly different
between conditions (P = 0.077, P = 0.811). Data suggests venous congestion at −12◦
HDT. Since subjects felt comfortable with lying in −12◦ HDT under our experimental
conditions, this posture only moderately deteriorates sleep. Obviously, the human body
can almost compensate the several fold effects of gravity in HDT posture like an affected
CSF circulation, airway obstruction, unusual patterns of propioception and effects on the
cardiovascular system.