ASEN 5016 Lecture 6b: Chronobiology

OBJECTIVES

1. Describe basic processes associated with circadian rhythm (CR)

2. Explain effects of space flight on CR

1. Fundamentals of circadian rhythm (terms and definitions)

Circadian - taken from Latin words meaning "around" and "day"

Circadian Rhythm - a self-sustained biological rhythm which in an organism's natural environment normally has a period of approximately 24 hours

CR Regulates body temperature, heart rate and hormone/enzyme cycles

Two strategies for establishing CR

- direct response

- anticipation

CR set by endogenous oscillators – rigid and slowly adapting

Entrained vs. free running cycles

Day/night cycles can entrain endogenous oscillators and mask overt rhythms

Endogenous oscillators in turn induce overt rhythms

When day/night signals become out of synch with endogenous signals, CR problems encountered

Circadian pacemaker in humans – Suprachiasmatic Nucleus (SCN)

(Optic Lobe in insects – studies have shown that CR rhythm maintained with single neuron in vitro, frequency stabilized with critical mass, ablation causes arrhythmicity, partial lesion modifies, transplant restores)

Most chronobiologists study circadian rhythms, endogenous cycles of behavior, or biological activity with a period of 24 hours.

Circadian rhythms, like the sleep-wake cycles discussed later, are generated by an internal clock that is synchronized to light-dark cycles in the environment and other daily cues.

Circadian rhythms are frequently plotted on an actogram. An entraining agent (an environmental time cue that has the ability to reset a biological clock, for example, exposure to bright light) can cause a phase shift whereby the activity is started earlier or later in the day.

Sleep and CR are fundamental regulatory processes of the nervous system and serve to regulate a wide range of functions

Physiological - body temp, cardiovascular activity, respiration and immune response

Hormonal – growth hormone, melatonin, cortisol, thyroid

Behavioral – movement, posture, reaction time

Cognitive – fatigue, alertness, vigilance, memory

Sleep deprivation à Performance deterioration and fatigue

Free Running Sleep-Wake cycle experiments with humans: Cave Studies

1991 – conditions of social and temporal isolation lead to ~25 hr circadian period

1993 – Compressed Spring hypothesis*: 25 hr CR constrained to 24 hr day/night cycle

1994 – spontaneous circadian day/night cycle is ~25 hours, slightly longer than body temp cycle, but the 2 coincide when subjects synchronized to environment

1999 – study suggests CR averages 24.18 hours…

Scheduling, clock available, leisure time, etc. – variables in studies

*Compressed Spring hypothesis – work-rest cycle is a ‘Zeitgeber’ in temporal isolation

If human CR is 25 hr but entrained to 24 hr, then cycle is unstable with periodic ‘resets’ over time

2. Effects of space flight on of circadian rhythm

Jet lag is an example of Circadian pattern interruption

Symptoms – depression, hopelessness, boredom, irritability, withdrawal, insomnia and fatigue

Pre- and post-flight CR shifting protocol

Space Flight Sleep Factors à SMS, stress, disorienting, 90 minute day/night cycles, background noise, dual shift operations

Evidence that immune system and metabolism can be affected by the amount of sleep we get

Desynchronization in space missions has been documented

Stress sleep cycle (sleep deprivation) – 2 to 3 hours in duration with little or no REM

REM sleep typically does not deviate much (~2 per 8 hours)

Most sleep disturbances found to occur near first and last days à SMS, noise and excitement

Overview of BION and Mir flight results – (monkeys and beetles)

Research using scorpions