Researchers are still choreographing the precise steps in the dance between stress and sex hormones, but these basics are clear: Sleep deprivation pumps up stress hormones like cortisol and norepinephrine, and when your body is producing more of those, it's unable to pump out your usual testosterone. Working too long and hard, overtaxing your adrenal system day after day – same effect. "The animal who is extremely anxious is not going to reproduce anymore," Van Cauter says, "and the stressed person isn't going to have the energy or desire to have much sex."
In the jungle that is competitive urban life, it may be that having a steady mate is the best protection against being unmanned by stress. James Coan, who runs a neuroscience lab at the University of Virginia, says the simple act of holding hands lowers stress and anxiety. Framingham State sociologist Virginia Rutter, coauthor of The Gender of Sexuality, says the reassuring and comforting qualities of a relationship can help you block anxiety and distraction, and focus on what's arousing in the here and now – the secret of a healthy libido. "When people feel safe, they get down to business," she says. Certainly, when it comes to the hydraulics of male physical arousal, relaxation is key. The parasympathetic nervous system takes over, allowing the smooth muscles in the lining of the penis to relax and the penile chambers to fill with blood. In an era when sex and sexual desire seem to be under attack on multiple fronts, it could be the best protection for lust is love.