Young Man Sleeping in Bed [© Christoph Wilhelm\zefa\Corbis](Young Man Sleeping in Bed [© Christoph Wilhelm\zefa\Corbis] )

Sex and sleep

According to Dr. Billy Goldberg, co-author of Why Do Men Fall Asleep After Sex?, there is little direct evidence explaining why men fall asleep. However, the chemicals oxytocin, prolactin, gamma amino butyric acid (GABA) and other hormones all contribute to “that roll-over-and-snore feeling” because they facilitate sleep.

“A man’s body chemistry changes after orgasm. The biochemical prolactin is released, physically altering his body and making him very tired,” says Dr. David McKenzie, a sex therapist in Vancouver, Canada. 

Further, exertion during sex and after climax depletes the muscles of energy-producing glycogen, which leads to sleepiness. Since men have more muscle mass than women, they’re generally sleepier after sex.

Men’s libido goes up and down

Think PMS is only for women? Think again. Your monthly peaks and valleys are triggered by changes in testosterone that affect your mood, libido, energy level, beard growth and sperm count.

According to research by naturopathic physician Dr. Marcus Laux, men have more energy, a greater sense of well being, lower body weight and less need for sleep during the peak of their cycle. The valleys bring apathy, indifference and the tendency to magnify small problems into big ones.

“If you keep track of your personal cycles, whether it’s shifts in energy levels, mood or sex drive, you can anticipate changes,” says Laux. “Then, you can take advantage of the times you’re at your prime and better cope when you’re not feeling your best.”

Your penis is a barometer of overall health

Dr. Mehmet Oz, co-author of You: The Owner’s Manual, calls it your “dipstick” because it reveals the health of your other body parts. The physical mechanisms that send blood to your brain, heart and kidneys also inflate your penis. If it’s not standing tall, you may have arterial problems.

“A man’s lack of interest in sex is a big red flag that something is out of balance. Up to 25 percent of men have a low-to-no sex drive,” says Dr. Laux. “It could be even higher, as men don't talk about low libido.”

The possible causes of restricted blood flow include diabetes, endocrine disorders, heart and vascular problems or kidney diseases. “But,” says Laux, “don't rule out fatigue, psychological factors, relationship issues, poor nutrition, insomnia or lack of exercise.”