
"My wife asked why there is always a puddle of pre on my stomach or a big wet spot on the sheets in the morning. I get all the random boners during the day and don't leak. Why does my dick leak so much at night?"
It could be from mini-ejaculations. Or it could be Pre-ejaculate. Pre-cum, is produced by the Cowpers Glands directly adjacent to the Prostate. Although the amount produced varies from guy to guy, it is still tied to the level of arousal. The more aroused, and the longer the arousal lasts, the more pre is sent along to prepare the way for your swimmers.
As we age and hormones level out we don't notice those random erections as much during the day. But, we're usually completely unaware of nighttime boners. Those can last anywhere from a few minutes to more than an hour. The level of hardness (arousal) can vary, too. But the ones at night often bring on sexual dreams triggering the resultant flow of pre-cum. Just like the full-on wet dream, pre-cum during sleep is considered a "nocturnal emission."
But pre-cum is a good thing. Can you say, "Free lube"?
According to Healthline, "Most men and boys don’t need any treatment for nocturnal emissions. They usually become less frequent as you move into your 20s. They may be more frequent, however, during periods in which you’re having less sexual intercourse or are masturbating less often.

"Increased sexual activity may lead to a decrease in nocturnal emissions. If you have questions about nocturnal emissions, talk with your doctor."
You could go into the long-winded explanation of random Blood Oxygen Boners (BOEs), morning wood, etc, but just tell your wife that dreaming about her gets you so excited it makes your cock drool. You could tell her the best way to decrease the pools is by having more sex.
She gets clean sheets, you get more sex. Look at that, a win-win!