When people say the sex was forgettable, this isn't what they meant.

It's called transient global amnesia, generally strikes during or just after sex, and can temporarily wipe out recent memory. It's also relatively rare — affecting five people per 100,000 — and little understood. 

"Out of the blue, totally healthy patients will be unable remember anything you told them," says Dr. Steven Lewis, a neurologist at Rush-Presbyterian-St. Luke's Medical Center in Chicago. 

It lasts a matter of hours and is ultimately harmless, he says. "But if I developed TGA right now, the next six hours would be gone forever." 

Who could imagine having sex and not remembering a thing? (Insert your own Bill Clinton joke here.) 

Where Am I? And Why Am I Naked? 

"We found two patients who had experienced TGA after sexual intercourse," says Dr. Lawrence Gardner of Johns Hopkins Medical School, who wrote about the unrelated cases in The Lancet

The two men, both in their 70s, were brought in by their concerned wives. About 30 minutes after sex, the men became confused and disoriented. It was hours before they snapped out of it. When they came to, they remembered nothing. 

Medical literature is full of similar cases. Like about 30 percent of TGA patients, one British man found his memory repeatedly wiped out. In the middle of sex with his wife, he would suddenly have no idea what was going on or where he was. The same happened with a man in Japan, according to a study from researchers at the University of Tokushima School of Medicine. 

Beware the Valsalva Maneuver 

Sex isn't the only trigger for TGA. It can also be caused by other types of exertion, immersion in cold water and severe emotional distress. Experts have theorized that TGA may be related to seizures, migraines and strokes, but Lewis has hypothesized another culprit: the Valsalva maneuver. 

It's not an exotic sexual technique. The Valsalva is the hold-your-breath and strain maneuver common to weight-lifters, and Lewis thinks it may divert blood from the memory-forming temporal lobes in the brain. 

"Something is going on that triggers TGA," Lewis said. "My theory is that the Valsalva maneuver is behind it." 

The risk of getting TGA is pretty low. But just to be safe, the next time you and your partner are dancing in the sheets, try to breathe a little and don't strain so hard. 

And if the concept of TGA is appealing — sex you can't recall — you might want to take a hard look at your relationship. Losing yourself in sex can be wonderful, but there are certain things you should try to remember.