Circadian rhythm
In humans, the light-dark cycle primarily synchronises the central clock; neural and humoral signals coordinate peripheral clocks. Core body temperature and cortisol are context-dependent physiological markers, whilst chronotype describes physiological phase differences. It follows that there is no universally superior training time window.
Auch bekannt als: Zirkadianer Rhythmus, Biologische Uhr, Circadian Rhythm
What a circadian rhythm is
A circadian rhythm refers to a temporal pattern of biological functions that is roughly aligned with the day. In the mammalian circadian system, there is a central clock, the suprachiasmatic nucleus (SCN), and peripheral clocks in almost every cell in the body. Light signals from the retina synchronise the central clock; neural and humoral signals – that is, signals mediated by soluble substances in the body – coordinate local oscillators in organs and tissues.
The biological clock is therefore an organisational system rather than a single measurement. Changes in core body temperature or cortisol levels may coincide with the time of day, but on their own they do not prove either the underlying rhythm or long-term adaptation.
How central and peripheral clocks are synchronised
In humans, the periodic light-dark cycle is the dominant environmental clock. Light signals from the environment can shift the phase of the circadian system. The nature of the response depends on the timing, intensity, duration and spectral properties of the light.
A blanket statement that morning light or blue light synchronises the body’s internal clock regardless of these conditions is therefore too simplistic. No general rule regarding light applies to everyone.
Which bodily rhythms become apparent in everyday life
Core temperature is a mixed indicator
Core body temperature fluctuates throughout the day. Sleep, wakefulness and activity alter or mask this rhythm. Around the time of the lowest temperature and during the rising phase, thermoregulatory reflexes are activated more slowly than around the time of the highest temperature and during the falling phase.
Core body temperature can therefore influence the interpretation of performance at different times of day; however, under everyday conditions, it is not a pure marker of the circadian clock. It is not possible to derive universal times for minimum or maximum performance, nor a simple causal rule whereby muscle performance follows temperature.
The morning rise in cortisol levels is not straightforward
An observational human study examined 201 healthy volunteers under everyday conditions using ambulatory microdialysis. It found no evidence that the rate of change in cortisol release during the first hour after waking was higher than in the hour preceding it. The absolute concentration and the dynamics differed significantly between individuals.
This negative finding neither refutes any measurement of a morning rise in cortisol nor a circadian cortisol rhythm. However, it does not provide direct evidence to support any training-related conclusions.
Further rules regarding hormones and performance remain unresolved
Fixed general times for testosterone or growth hormone, the claim that catabolic activity peaks in the morning, and a causal cortisol-to-testosterone ratio affecting strength or muscle growth cannot be deduced from the existence of a circadian clock. There is no direct human endpoint for these relationships.
What the chronotype actually describes
A human study involving 68 younger and 40 older participants categorised the participants according to their self-reported preference for mornings or evenings. Among the younger ‘morning types’, the phases of the melatonin and core body temperature rhythms occurred earlier than among the ‘evening types’. The relationship between circadian phase and sleep-wake timing varied with age.
A chronotype therefore describes a link between a preferred time of day and a physiological phase; it does not automatically imply a fixed shift for everyone. The study finds neither a consistent difference of two or three hours nor any benefit to strength training at a specific time of day.
What can be deduced for strength training
There is no directly applicable human endpoint in trained strength athletes for the specific combination of chronotype, circadian phase, light, core temperature and hormone profiles with acute strength performance, as well as long-term strength or hypertrophy adaptations. No general optimal training window or fixed percentage change in strength can be derived from this unproven causal chain.
- Treating the time of training as context: Sleep-wake cycles and activity levels influence how core body temperature varies throughout the day. These factors should be taken into account when interpreting comparisons across different times of day; they do not, on their own, indicate a superior time for training.
- Don’t make chronotype a cure-all: A circadian phase that is earlier or later in the cycle is a physiological phenomenon. In the absence of a specific training objective, this does not lead to a general recommendation to always schedule training for the morning or the evening.
- Distinguishing between acute and long-term endpoints: A momentary response in temperature or cortisol does not answer the question of strength gain or hypertrophy. These endpoints must be examined separately in studies and under comparable conditions.
Classification
The robust finding is a causal chain in the organisation of time: light synchronises the central clock, which in turn coordinates peripheral clocks, whilst physiological markers are also influenced by the sleep-wake cycle, activity, light conditions and age. This provides a plausible biological explanation for time-dependent physiological differences, but does not constitute a general training guideline. The causal chain alone does not prove either a specific training time window or provide a causal explanation for long-term strength or muscle adaptation.
Sources
- Dibner C et al. The mammalian circadian timing system: organization and coordination of central and peripheral clocks. Annu Rev Physiol. 2010. PubMed
- Duffy JF et al. Entrainment of the human circadian system by light. J Biol Rhythms. 2005. PubMed
- Klaas S et al. Awakening not associated with an increased rate of cortisol secretion. Proc Biol Sci. 2025. PubMed
- Waterhouse J et al. The circadian rhythm of core temperature: origin and some implications for exercise performance. Chronobiol Int. 2005. PubMed
- Duffy JF et al. Relationship of endogenous circadian melatonin and temperature rhythms to self-reported preference for morning or evening activity in young and older people. J Investig Med. 1999. PubMed
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