The last time I sweated through my shirt, I vowed that it would never happen again. Sweat shame had dogged me for too many years. No longer would armpit puddles dictate the color of my blouse. Never again would I twist underneath a hand dryer to dry my damp underarms. It was time to try clinical-strength antiperspirant.
The one I bought looked like any old antiperspirant, a solid white cream encased in a plastic applicator. But its instructions seemed unusual: “For best results, apply every night before bed and again in the morning.”
Every night?
I swiped it across my armpits before bed, and to my surprise, they were dry all the next day. I kept poking them in disbelief—deserts. But I would later discover that there isn’t anything particularly special about this product. Nighttime application improves the effects of any traditional antiperspirant, including those combined with deodorant (the former blocks sweat while the latter masks smell). Research has shown this for at least 20 years; none of the experts I spoke with disagreed. Yet many of us swipe our armpits in the morning before we head out for the day. Somehow, Americans are trapped in a perspiration delusion.
Putting on antiperspirant in the evening feels roughly akin to styling your hair right before bed. Both are acts of personal maintenance that people take not only for their own well-being but also in anticipation of interactions with others. This idea is reinforced by ads for antiperspirants, which tend to feature half-dressed actors getting ready in bathrooms or changing rooms; see, for example, the Old Spice guy. These ads also tend to mention how long their products work—24 hours, 36 hours—implying that their effectiveness starts to fade once they are applied. In a recent Secret commercial, a woman rolls on antiperspirant in a daylit bathroom, then scrambles to make her bus, relieved that she is prepared for such sweaty moments for the next 72 hours.
What these ads don’t say is that these products need the right conditions to work effectively. Antiperspirant isn’t a film on the surface of the armpit that stops moisture from leaking through, like a tarp over wet grass. Instead, it functions like a bunch of microscopic champagne corks, temporarily sealing sweat glands from spraying their contents. The active ingredient in most antiperspirants is some form of aluminum salt, compounds that combine with moisture on the skin to form “gel plugs” that dam up the sweat glands. These gel plugs prevent not only wetness but also odors, because bacteria responsible for foul smells thrive best in moist (and hairy) conditions, according to Dee Anna Glaser, a dermatologist and board member of the International Hyperhidrosis Society, a group that advocates for patients with excessive sweatiness.
Gel plugs are finicky. They need a little bit of sweat in order to form—but not too much. Antiperspirant applied in the morning isn’t ideal, because people sweat more during waking hours, when they’re active. If the armpits are too sweaty in the hours after application, the product gets washed away before it can form the plugs. The body is cooler and calmer during sleep. For gel plugs to form, “baseline sweating is optimal at nighttime before bed,” Glaser told me. Nighttime application has been shown to increase the sweat-reduction ability of normal antiperspirant from 56 percent to 73 percent.
But wait, I can already hear you thinking, what happens if I shower in the morning? Here’s the thing: Antiperspirant lasts through a shower. “The plugs won’t wash away much,” even though the residue and scent probably will, Mike Thomas, a former scientist with Procter & Gamble and an advocate for the International Hyperhidrosis Society, told me. After 24 hours or more, the plug naturally dissolves. Reapplying antiperspirant during the day can be beneficial, Shoshana Marmon, a dermatology professor at New York Medical College, told me. Still, it works best if applied to dry armpits that, ideally, stay dry enough for the plugs to form. For most people, Marmon added, putting it on “clean, dry skin at night” provides enough protection to last through the next day.
Again, none of this information is new or hard to find. One of the earliest studies demonstrating the value of nighttime application was published in 2004; it showed that applying antiperspirant in the evening, or twice daily, was significantly more effective than morning-only use. Indeed, the stance of the American Academy of Dermatology is that it’s best to put antiperspirant on at night. Media outlets have covered this guidance since at least 2009.
For the perpetually sweaty, discovering this guidance only now, after decades of embarrassing photos and ruined shirts, might spark belief in a grand conspiracy: They don’t want you to know the truth about armpit sweat. Indeed, it isn’t mentioned on the labels of most regular-strength antiperspirants. The reasons for this are more banal than nefarious. Most people don’t sweat excessively, so applying antiperspirant the usual way is sufficient. “Manufacturers may keep instructions simple to fit general habits, so the idea of using antiperspirant at night doesn’t always make it into mainstream awareness,” Danilo C. Del Campo, a dermatologist at Chicago Skin Clinic, told me. The difference between antiperspirant and deodorant still eludes many people and, in fact, may bolster the insistence on morning application. Deodorant is essentially perfume and has no impact on sweat production. It’s “best applied when odor control is most needed, typically in the mornings,” Marmon said.
When I asked brand representatives why so many antiperspirants don’t mention nighttime use in the directions, they pointed to the potential for confusion. “It’s a bit counterintuitive for people to use antiperspirant at night, because most people think of applying it as part of their morning routine,” Maiysha Jones, a principal scientist at P&G North America Personal Care, which owns brands such as Secret and Old Spice, told me. But, she added, it is indeed best to use it at night. “Antiperspirants are commonly assumed to be a morning-only product and applied during the morning routine,” Megan Smith, a principal scientist at Degree Deodorant, told me.
In other words, people are used to applying antiperspirant in the morning because companies don’t tell them about the nighttime hack … but companies don’t tell them because people are used to putting it on in the morning. Omitting helpful instructions just because they might be confusing isn’t doing America’s perspirers any favors. Anyone who’s ever experienced an overly moist underarm can surely be coaxed into shifting armpit maintenance back a measly eight hours. People go to far greater lengths to self-optimize, whether it’s teens adopting multistep skin-care routines, or wellness bros taking dozens of supplements.
The science is well established, and the guidance is clear. But the ranks of nighttime swipers may not increase immediately. Routines have to be reset, assumptions picked apart. Some evenings, I find it exhilarating to buck the orthodoxy of personal hygiene. Other nights, it gives me pause. Applicator hovers over armpit, brain stumbles on belief. Will this really last past the sunrise, through a shower, beyond the hustle of the day? Even after learning about the science, “some people just don’t believe,” Thomas said. All there is to do is try. In go the corks, out go the lights.
