The Neurolab mission (STS-90) launched on April 17, 1998, and landed at Kennedy Space Center on May 3, 1998. The UCSD Physiology/NASA Lab project for Neurolab was called "Sleep and Respiration in Microgravity." The UCSD team was paired up with a team from Brigham and Women's Hospital at the Harvard Medical School. Together, they studied the quality of sleep in microgravity, and the effect of melatonin on sleep. The contribution of alterations of the control of ventilation to sleep disturbance was also examined by the "Sleep Team." The UCSD group also studied the lung function of the crew while awake, performing several pulmonary function tests.
There is evidence that sleep is affected by microgravity. However, despite anecdotal reports of poor quality of sleep in microgravity, and the common use of mild sedatives to improve the quality of sleep in the Space Shuttle, there had been no detailed studies of sleep in microgravity. In many people, nocturnal hypoventilation leads to hypoxemia (low oxygen), hypercapnia (high carbon dioxide), and is a potent arousal stimulus. During sleep many people experience periodic breathing, and there is one report of sleep apnea actually occurring in flight aboard the Russian Space Station Mir. Possible changes in the chemoreceptive control of ventilation brought about by exposure to microgravity may well contribute to alterations in the sleep pattern in microgravity.
The Sleep Team measured respiration during sleep in microgravity by instrumenting subjects with a Respiratory Inductance Plethysmograph (RIP) and pulse oximeter, allowing continuous measurement of the motion of both the rib cage and abdomen as well as arterial oxygen saturation. In addition, subjects were fitted with an EEG, EOG, a temperature sensor allowing us to determine sleep stage, and with an ECG. From these sensors, we can determine changes in ventilation, relative rib cage and abdominal contribution to ventilation, thoraco-abdominal asynchrony, sympathetic and parasympathetic contributions to heart rate variability, and the coupling between respiration and heart rate, all as a function of sleep stage. This data analysis is being done in collaboration with Manuel Paiva and the Biomedical Physics Laboratory, Université Libre de Bruxelles, Brussels, Belgium. There is strong evidence that there are neurological changes in the cardiovascular system brought on by exposure to microgravity, and we expect to find that there will also be changes in the neurological control of ventilation in microgravity. We expect that these will manifest themselves as changes in the pattern of sleep.
In addition, we studied the neurological control of ventilation by measuring the ventilator's response to both hypoxia and hypercapnia using our Astronaut Lung Function Experiment (ALFE) hardware. Preflight, inflight, and postflight, we measured the quasi-isocapnic hypoxia response and the hypercapnic rebreathing response. We also measured cardiac output, diffusing capacity lung water, and resting oxygen consumption. These are supplemented by RIP and pulse oximetry measurements allowing determination of respiratory timing without the interference of a mouthpiece, as well as arterial oxygen saturation. This will provide us information regarding the change in ventilatory control and the ventilatory-baroreceptor integrated reflex.
The combination of sleep studies and the awake measurements performed on the same subjects in microgravity will shed considerable light on the changes in the neurologic control of ventilation that occur when gravity is removed.
For more information about the Neurolab mission, please see these Neurolab pages:
To view the E198 team and Neurolab crew at work, choose to view pictures of Baseline Data Collection, Preflight hardware testing at KSC, or Preflight Training.