We now know that humans’ mental and physical abilities vary greatly depending on the time of day – due to light-driven physiological processes called circadian rhythms – and the time of year – due to circannual rhythms.
According to researchers from the University of Zurich, the phenomenon of time-dependent performance also holds true for individual cells within us; well, one very important type of cell at least – sperm.
Writing in the journal Chronobiology International, the team of reproductive biologists report that semen produced early in the morning – before 7.30am – show significantly higher sperm concentrations, total sperm count, and percentage of sperm with normal morphology than semen produced at other times of day.
“Regarding seasonal variations, we found a significant increase in sperm concentration and total sperm count in spring, whereas we noticed a significant reduction in summer,” they stated.
These findings, which could help improve the success rate of both natural and assisted conception attempts, came from analyzing 12,245 samples collected from 7,068 men, all of whom were patients at University Hospital Zurich's reproductive endocrinology department between 1994 and 2015.
Curiously, lead author Min Xie and colleagues observed that sperm motility does not appear to be impacted by time of day in men who produce healthy sperm, yet men whose swimming gametes were abnormal (low motility, concentration, morphology, or a combination of the three) had the best motility between 8.30 and 10.00am. No association between motility and season was found across the group as a whole or any abnormal subgroups.
Of course, because this study only assessed sperm from men living in one region of the world, the results should be confirmed in a more diverse group before we assume the pattern is applicable to males everywhere.
In addition, as the authors themselves concede, the outcomes may have been impacted by differences in the men’s health statuses and lifestyles (obesity and smoking, for example, are known to alter male fertility), but since this data was not collected, they cannot adjust for it.
It’s also possible that sperm are more active at more extreme times of day, but the researchers only collected samples of semen that were, ahem, expelled during the 12 hours the clinic was open.
“Since our data were collected over a period of 12 hours beginning in the early morning, we lack data on the (late) evening, i.e. when sexual activity is highest,” they wrote.
The team hopes that future follow-up studies will examine the mechanisms underlying how signaling molecules key to circadian rhythms, such as melatonin and cortisol, alter sperm production.
In conclusion, couples who are hoping to conceive shouldn't alter the timing of their attempts just yet, but taking advantage of the man's natural morning endowment may not be a bad idea.