Gyllenhaal features in Prada's new ad campaign for fragrance Luna Rossa Ocean, and sat down with Vanity Fair recently to talk about 'all things aquatic', in the theme of the new product.
Gyllenhaal was asked about his experience of 'aquatic New York', with journalist Laura Regensdorf posing: "I feel like New Yorkers forget that this is a water town. What are your defining water experiences here? Do you run along the river? Have you had a hilariously bad date on a ferry?"
It was at this point that Gyllenhaal directed the focus to the bathroom.
He said: "Well, mostly it's through showering. It's mostly a bathing interaction. This is not actual water, like the Hudson, but there was that great spa, Aire, that has water underground. I went there a couple of times.
"What other water interactions have I had? I'm a pier kind of guy. I enjoy the piers. But I haven't spent a lot of time, admittedly, on the Hudson itself. Maybe farther [upstate], but not here. Sorry to disappoint."
He was then asked if there was anything 'revelatory' about his shower ritual, to which he replied: "I always am baffled that loofahs come from nature. They feel like they've been made in a factory but, in fact, it's just not true. Since I was young, it's amazed me.
"More and more I find bathing to be less necessary, at times. I do believe, because Elvis Costello is wonderful, that good manners and bad breath get you nowhere. So I do that.
"But I do also think that there's a whole world of not bathing that is also really helpful for skin maintenance, and we naturally clean ourselves."
And Gyllenhaal is not alone, as it turns out the idea is something of a bizarre celebrity trend.
Mila Kunis and Ashton Kutcher recently said they only gave baths to their kids - six-year-old Wyatt and four-year-old Dimitri - when they could 'see dirt on them'.
The couple appeared on the Armchair Expert podcast, where Kunis told hosts Dax Shepard and Monica Padman: "When I had children, I also didn't wash them every day.
"I wasn't that parent that bathed my newborns - ever."
Kutcher added: "Now, here's the thing: If you can see the dirt on them, clean them. Otherwise, there's no point."
Shepard, who is married to actor Kristen Bell, had also admitted a similar approach, saying: "We bathed our children every single night prior to bed as their routine, then somehow they just started going to sleep on their own without their routine and we had to start saying [to each other] like, 'Hey, when was the last time you bathed them?'"
Appearing on The View, Bell also once explained: "I'm a big fan of waiting for the stink.
"Once you catch a whiff, that's biology's way of letting you know you need to clean it up.
"There's a red flag. Honestly, it's just bacteria; once you get bacteria you gotta be like, 'Get in the tub or the shower'."
I'll be honest, it kind of sounds like they're all onto something.