If you wear antiperspirant deodorant, you probably put it on the same way everyone else does: right after your morning shower. That's what all the bright, airy commercials show people doing! Turns out, that's probably the worst time to apply the stuff.
First, let's be clear: deodorants are not necessarily antiperspirants. Though pretty much everyone refers to the stuff as simply "deodorant" there is a difference; antiperspirants block you from sweating, deodorants cover up the smell and often include antiperspirant ingredients.
Antiperspirants work by physically blocking the sweat ducts in your arm—typically with an active ingredient. For example, Dove Clinical Protection uses Aluminum Zirconium Tetrachlorohydrex Gly (20%), along with a host of other inactive ingredients like fragrance and moisturizers.
The problem? If you roll on a product like this in the morning, especially if your armpit isn't dry yet, you're not going to get the best protection. In the morning, your sweat glands are likely to be more active, especially if you immediately head out the door, taking that active ingredient with it.
In order for antiperspirants to work, they need time to settle into your sweat ducts and do their thing. The best time to do this is right before bed when you're calm, cool, and least likely to be sweating. (I recommend a cool evening shower, but that's just solid life advice).
Do that, and antiperspirant should be effective through the next 24 hours. It won't get washed off in your morning shower, either.
Unfortunately, the same is not necessarily true of deodorizing ingredients. Those ingredients (like fragrance and alcohol) are more prone to wash off in your morning shower, so you may be better off applying antiperspirant at night and deodorant in the morning once you're clean and dry.
Confusing? Sure, but if it means your arms stay (and smell) clean and fresh, it's probably worth the hassle.