Duffy, Jeanne F. | Search NASA

NASA NTRS2002

Getting through to circadian oscillators: why use constant routines?

Overt 24-h rhythmicity is composed of both exogenous and endogenous components, reflecting the product of multiple (periodic) feedback loops with a core pacemaker at their center. Researchers attempting to reveal the endogenous circadian (near 24-h) component of rhythms commonly conduct their experiments under constant environmental conditions. However, even under constant environmental conditions, rhythmic changes in behavior, such as food intake or the sleep-wake cycle, can contribute to observed rhythmicity in many physiological and endocrine variables. Assessment of characteristics of the core circadian pacemaker and its direct contribution to rhythmicity in different variables, including rhythmicity in gene expression, may be more reliable when such periodic behaviors are eliminated or kept constant across all circadian phases. This is relevant for the assessment of the status of the circadian pacemaker in situations in which the sleep-wake cycle or food intake regimes are altered because of external conditions, such as in shift work or jet lag. It is also relevant for situations in which differences in overt rhythmicity could be due to changes in either sleep oscillatory processes or circadian rhythmicity, such as advanced or delayed sleep phase syndromes, in aging, or in particular clinical conditions. Researchers studying human circadian rhythms have developed constant routine protocols to assess the status of the circadian pacemaker in constant behavioral and environmental conditions, whereas this technique is often thought to be unnecessary in the study of animal rhythms. In this short review, the authors summarize constant routine methodology and what has been learned from constant routines and argue that animal and human circadian rhythm researchers should (continue to) use constant routines as a step on the road to getting through to central and peripheral circadian oscillators in the intact organism.

Non-NASA Center↗

NASA NTRS1991

Research on sleep, circadian rhythms and aging - Applications to manned spaceflight

Disorders of sleep and circadian rhythmicity are characteristic of both advancing age and manned spaceflight. Sleep fragmentation, reduced nocturnal sleep tendency and sleep efficiency, reduced daytime alertness, and increased daytime napping are common to both of these conditions. Recent research on the pathophysiology and treatment of disrupted sleep in older people has led to a better understanding of how the human circadian pacemaker regulates the timing of the daily sleep-wake cycle and how it responds to the periodic changes in the light-dark cycle to which we are ordinarily exposed. These findings have led to new treatments for some of the sleep disorders common to older individuals, using carefully timed exposure to bright light and darkness to manipulate the phase and/or amplitude of the circadian timing system. These insights and treatment approaches have direct applications in the design of countermeasures allowing astronauts to overcome some of the challenges which manned spaceflight poses for the human circadian timing system. We have conducted an operational feasibility study on the use of scheduled exposure to bright light and darkness prior to launch in order to facilitate adaptation of the circadian system of a NASA Space Shuttle crew to the altered sleep-wake schedule required for their mission. The results of this study illustrate how an understanding of the properties of the human circadian timing system and the consequences of circadian disruption can be applied to manned spaceflight.

Czeisler, Charles A.↗