FOR SOME RUNNERS, running long distance results in overheating, sleepiness and raised temperatures for hours after their shoes have been kicked off.
High post-run temperatures are not unusual. Many runners finish longer runs with rectal temperatures on the 37 to 28 degree range, and some notable marathoners have finished races with rectal temperatures of 41 degrees without any noticeable signs of heat stroke.
So why do some runners get sleepy after long runs? I am not sure I have a great explanation, although I do notice somnolence after long inline skate or Nordic ski workouts; the 1.5 to 2 hour variety, as opposed to the 30 to 60 minute sessions. This has become more prevalent as I have aged.
From what I have been able to read, exercise like brisk walking for 30 to 60 minutes tends to make people feel alert and energetic. More intense exercise like running and longer duration endurance activity tends to make people sleepy, and this becomes more pronounced with age.
There are varying hypotheses regarding the mechanism behind the post-exercise somnolence, including waning hormone levels (testosterone in men) with aging and release of cytokines during exercise. Two of the likely culprits are interleukin 6 and tumor necrosis factor alpha, which make people drowsy, prolong the sleep time, and are both released by muscle during exercise.
Increased sleepiness has been reported in many prominent distance runners, with runners sleeping more with heavy training and long races. I am not sure if a cool shower will counter the sleepiness, but it is worth a try. If it is interfering with your work or other activities, you might try training later in the day and using exercise to induce night time sleep.