Seasonal & Circannual Testosterone Variation
How Daylight and Season Shift Male Androgen Levels
Evidence grade 3 of 5, Mixed. Read how the Atlas grades evidence.
The written rationale for this grade is still being prepared.
- FDA
- Physiological phenomenon, not an intervention
Reference only. Not medical advice.
This is something to take to a clinician. The Atlas can tell you what is known about seasonal & circannual testosterone variation in general. It cannot tell you what is happening in your body, and the difference matters most for exactly the symptoms that bring people to this page.
Male testosterone shows a documented circannual rhythm layered on top of its well-known diurnal (morning-peak) rhythm. Multiple cohorts in industrialized populations report higher levels in late summer and autumn and a nadir in winter to early spring. Effect sizes are real but modest, and they matter clinically because the season and time of a blood draw can move a man across a diagnostic threshold.
Testosterone is not a fixed number; it drifts with the calendar.
What people take it for
These are the reasons people give for taking it. They are not effects it has been shown to have.
How it is supposed to work
The HPG axis is sensitive to photoperiod and likely temperature. Day length signaling, mediated in part through melatonin and the suprachiasmatic clock, modulates gonadotropin and downstream testosterone output across the year, producing population-level seasonal swings. The mechanism is not fully resolved and the direction of seasonal peaks has varied between studies and latitudes.
What people get wrong
Often saidA single low test means you have low testosterone
ActuallyA lone reading can be misleading. Season and time of day both shift levels; guidelines call for a morning draw and confirmation on a second sample.
Often saidWinter low T means something is wrong with you
ActuallyA modest winter dip is a normal feature of the circannual rhythm in many men, not necessarily pathology.
Who profits, and who pushes back
Nobody is named here yet, because we have not done that work on this entry. Read that as unfinished. Somebody almost certainly sells this, and somebody almost certainly disputes it. Assume both until we have checked.
At a glance
- Typical peak
- Late summer to autumn
- Typical nadir
- Winter to early spring
- Layered on
- Diurnal morning-peak rhythm
What this entry does not yet cover
What follows is unfinished work on our side, not a finding about Seasonal & Circannual Testosterone Variation. A short entry is not a safe entry.
- Timeline. We have not built the dated record of research and regulatory events.
- Cost and market. We have not documented what it sells for or who sells it.
- Supply chain. We have not mapped where it is made.
- Who uses it. We have not documented the user groups.
- What could change. No forward-looking summary yet.
- How it compares. We have not set it against the alternatives.
- Children and teenagers. We have not checked what is known about younger people.
- What can go wrong. We have not written the risk line for this one. Read the safety section below rather than treating its absence as reassurance.
- Where we land. We have not taken a position on this yet.
Sources
The studies below are named and described, but not linked. You cannot open them from here and check our reading against them, which means this page is our summary of the literature rather than a document you can verify. Weigh it accordingly until the links exist.
The studies that matter
Reinberg et al., circannual rhythm in Parisian men1975
Plasma testosterone and sexual activity peaked around October in a small male sample
Smals et al., seasonal pattern in healthy men1976
Significant seasonal variation with summer to early-autumn peaks and winter to early-spring nadir
Large real-world database studies2019-2023
Confirmed total and bioavailable testosterone peaking in August-October with a nadir around late winter across thousands of men