Don't be too quick to blame a lousy lover or too much tequila if your last tango between the sheets was just plain forgettable.
You might simply be suffering from a rare and - Bill Clinton jokes aside - very real case of sexual amnesia.
Doctors say it's a version of TGA, short for transient global amnesia. It strikes during or right after sexual intercourse, usually wiping out memory for six hours or more. For better or for worse, that whole time period is lost - forever.
"It's an extremely scary event," says Dr Lawrence Gardner, a haematologist at the Johns Hopkins Medical School in Baltimore who wrote about the baffling phenomenon in The Lancet, the British medical journal.
Gardner and his colleague, Dr Chi Dang, saw two patients, both men in their 70s, who were stricken after sexual intercourse with their wives.
Both victims were otherwise healthy. Within 30 minutes of intercourse, however, both became so disoriented and confused that they couldn't even identify the president of the United States. One, a 75-year-old retired physician, babbled incoherently and wasn't back to normal for 15 hours. Even then, he couldn't recall anything that had happened in the six hours after having sex.
Like most patients, both men resumed sexual activity and have never had another attack, Gardner and Dang say.
But that's not always the case. Researchers at the University of Tokushima School of Medicine in Japan, for instance, report one patient whose memory was zapped during sex over and over again. Medical textbooks chronicle similar cases.
About one patient in five will have a repeat attack, says Dr Steven Lewis of Rush-Presbyterian-St Luke's Medical Centre in Chicago.
During an attack, Lewis explains, a patient stops making memories. It's just like typing a letter on your personal computer then neglecting to save it to the hard drive - lost forever. The patient also loses memory of events that occurred days, months, even years earlier, but it usually returns eventually.
"They'll repeat things over and over, and ask: 'What am I doing here?' But later, they won't even remember asking the question," Lewis says. "Usually, the wife will recognise that there's a problem and call the doctor, and it's later when you take the [medical history] that you realise what they were doing."
The risk is small
Fortunately, your risk of developing sexual amnesia is small.
Studies show it affects fewer than a dozen out of every 100 000 people. But baby boomers, beware. People in their 50s and 60s are most susceptible. The risk tapers off for older folks. And sexual amnesia is slightly more common among men - for reasons doctors are at a loss to explain.
So, is there any way to protect yourself - short of abstaining from sex?
"Not really. You can't put a condom on your brain," Gardner quips. "And you don't need a neurological exam before intercourse."
Besides, even celibacy is no sure safeguard.
Experts say migraine headaches, mini-strokes and seizures can all trigger transient global amnesia.
So can severe pain. Patients have suffered TGA after having teeth pulled or undergoing acupuncture. Researchers also write of amnesia attacks brought on by extreme emotional distress, such as the death of a loved one or witnessing a gruesome traffic accident.
Watch out for that water!
And that's not all.
Your memory also can be wiped out if you take a dip in water that's too cold or tackle any form of extreme physical exertion - from digging a car out of a snowdrift to laying concrete.
Which leads Lewis to a provocative theory about the cause of TGA, including the intercourse-induced variety. Writing in The Lancet, he blames something scientists call the Valsalva maneuver.
But don't get the wrong idea. The Valsalva manoeuvre is not some feat of sexual gymnastics. It's the breathe-in-and-bear-down action we all do automatically during extreme physical exertion - from weightlifting to wood chopping. It happens, too, when we're in extreme pain, swimming in too-cold water or having sex - the same activities that trigger TGA.
How? Lewis thinks the Valsalva manoeuvre temporarily siphons blood flow from the temporal lobes of your brain, where memories form.
"A credible hypothesis," says Dr. John R. Hodges, a British neurology professor known around the world for his pioneering amnesia research. "The [cause] of TGA has been a mystery since it was first described. Just as epilepsy has many underlying causes, it is likely that TGA is also multifactorial."
Just keep this in mind: Sexual amnesia - the real deal - doesn't happen often. And even when it does, it's no big deal. So relax.
"Patients are fine afterwards," Lewis says. "There is definitely no reason for people to avoid the triggers."