Shaving at night has nothing to do with the healing power of moonlight. In actuality, it's all about shaving your underarms when the skin has the most time to heal and rejuvenate.
During the day, your underarms accumulate a lot of sweat and bacteria. If you shave in the morning, you expose your recently shaved skin -- which is often raw and sensitive -- to an entire day's worth of bacterial invaders.
It's also common practice to apply antiperspirant or deodorant to your underarms right after shaving. This makes sense in the morning, because you want to protect yourself from excessive sweating and body odor during the day ahead. But as we mentioned earlier, recently shaved skin is highly sensitive to the chemicals in many antiperspirants and deodorants. So, applying deodorant directly after shaving will increase the likelihood of skin bumps, redness and irritation.
That's why some dermatologists recommend that you shave your underarms at night. This way, your underarm skin has time to repair itself from the shave before the onslaught of bacteria that comes from daytime sweating. It also allows the skin time to recover before you apply deodorant or antiperspirant in the morning.