An Introduction to Circadian Endocrine Physiology: Implications for Exercise and Sports Performance

Abstract

Biological rhythms that recur approximately every 24 h (circadian) are driven by the internal biological clock located in the suprachiasmatic nuclei (SCN) of the hypothalamus. These circadian rhythms are intrinsically generated but also synchronized by external time cues—the daily light/dark cycle being the most important. Preferably under controlled conditions (constant routine or forced desynchrony), appropriately timed measurement of the 24-h variation in certain hormones (e.g., melatonin) or other relevant physiological variable (e.g., core body temperature) can be used to evaluate circadian phase. Interactions between circadian rhythms and sleep have a significant effect on multiple physiological processes. Human performance (cognitive and athletic) exhibits a circadian variation with minimal levels around time points close to the core body temperature minimum (shortly after the maximum melatonin secretion) while peak performance is observed around time points close to the core body temperature maximum (shortly before melatonin onset). Furthermore, misalignment of the internal clock with the external environment as seen in jet lag and shift work can negatively affect athletic performance and appropriately timed exposure to and avoidance of bright light can be used to mitigate the effects of jet lag. The circadian variation in performance can affect outcome of competitions and could be also important for choosing the optimal time to exercise.

Glossary

Circadian phase

This is the phase of the circadian cycle, representing the timing of the internal biological clock and is usually estimated by measuring onset–offsets, troughs or peaks, or changes in slopes (i.e., increasing vs. decreasing) in circadian markers such as hormones (melatonin, cortisol, prolactin) or CBT. Circadian phase measurement should be performed in controlled dim light conditions. In animal research, rest–activity in dim light conditions is used to estimate circadian phase.

Diurnal (in relation to circadian)

Refers to variations in a physiological para­meter with time of day. In contrast with circadian variations which are usually endogenous (i.e., intrinsically generated by the body clock, see below) but influenced by external light–dark cycle, diurnal variations can be either driven by the biological clock, external light–dark cycle, the sleep–wake cycle, or their interaction.

Entrainment

The process by which biological rhythms are synchronized (by timing signals) to a 24-h environmental cycle (usually the day/night cycle). Under entrainment or entrained conditions, circadian rhythms usually oscillate with a period of 24 h, induced by exposure to time cues (i.e., zeitgebers).

Free-running rhythm

A non-24-h rhythm seen in the absence of timing signals, most importantly in dark (or very dim light) conditions, which expresses the intrinsic circadian period of the circadian pacemaker (the suprachiasmatic nuclei, see below). Almost all animals have the internal circadian period slightly different than 24 h, either slightly shorter (as in a majority of rodents) or slightly longer (as in humans, among other animals). Animals with a longer than 24-h circadian rhythms have a tendency to phase delay from one day to another in the absence of exposure to light during morning hours.

Jet lag

A circadian rhythm sleep disorder consisting of sleep difficulty at night, daytime sleepiness, impairment of daytime function, gastrointestinal disturbance, and general malaise associated with transmeridian air travel, resulting from a desynchrony between external time cues, sleep–wake, and timing of endogenous rhythms.

Masking

Obscuring of circadian rhythms (driven by the circadian pacemaker) by ­external or internal factors. For example physiological processes and behaviors (such as opening and closure of eyelids, activity, food intake) associated with the sleep–wake cycle, exposure to bright light, eating, drinking, and standing could all be related to the rhythm of a physiological variable, though the rhythm is primarily generated endogenously by the circadian pacemaker. Therefore, measuring the rhythm of the variable (e.g., CBT) in the presence of an intact sleep–wake cycle and all the other aforementioned external factors could “mask” the true contribution of the circadian pacemaker to the rhythm and lead to inaccurate measurements.

Morningness–eveningness (ME)

The natural tendency for an individual to either go to sleep and wake up early and perform best in the morning (a morning person) or to go to sleep and wake up late and perform best in the evening (evening person). The ME has been related to clock gene polymorphisms and to circadian sleep disorders.

Nadir

This is the lowest point of a biological rhythm, e.g., the nadir of CBT is the ­lowest point on the CBT rhythm and is usually around 2 h before habitual wakening time in most individuals with a stable circadian rhythm.

Phase advance

Positioning of a particular circadian rhythm earlier relative to clock time or other circadian markers.

Phase delay

Positioning of a particular circadian rhythm later relative to clock time or other circadian markers.

Phase response curve (PRC)

A graphical illustration of the relationship between the timing of exposure to a zeitgeber or other intervention (on the x axis) and the shifting induced by the exposure to the zeitgeber or other intervention. Conventionally, on the y axis positive values represent phase advances and negative values represent phase delays. For example the circadian phase-shifting effects (advance or delay, depending on the time of exposure) of bright light or melatonin administration on any marker of circadian rhythms (such as CBT or the nocturnal melatonin secretion measured in blood—or the onset of nocturnal melatonin secretion in saliva) have been presented as phase response curves. On the x axis, timing is usually measured in relationship to a circadian marker—such as core temperature trough or the onset of melatonin secretion (i.e., internal timing) rather than external timing. Determination of circadian phase response curves is very demanding in terms of funds and time—requiring highly controlled conditions and minimizing exposure to zeitgebers.

Postprandial dip (or early afternoon dip)

The dip in performance observed during the mid-afternoon hours (incorrectly called postprandial), because it occurs in anticipation and not as a physiological reaction to the main meal of the day. For most individuals, performance measures (physical and cognitive) exhibit an increase from a low at the morning wake time to peak levels in the early evening time, but in some individuals a dip in performance is observed during the mid-afternoon hours.

Suprachiasmatic nuclei

A group of brain cells located bilaterally above the optic chiasm in the anterior basal hypothalamus and demonstrated to be the site of the master circadian oscillator (“body clock”) that synchronizes, as a conductor does with the orchestra, circadian rhythms of peripheral tissues, organs, and cells.

Wake maintenance zone

This is the time of the day (usually in the late evening) when the propensity for sleep is lowest and wakefulness or arousal is increased. The wake maintenance zone (also referred to as the forbidden zone for sleep) is mediated by the circadian pacemaker.

Zeitgeber

The name given to any external time-signaling stimuli that help maintain periodic regularity in circadian rhythms. It is a German word which literarily means “time giver.” Light is considered the most potent zeitgeber. Other zeitgebers are exercise, food, temperature, and social interactions.