Sedating antihistamines can mess with your light therapy timing, but nasal steroids are fine.

For most people, antihistamines and nasal steroids don’t directly mess with the mood benefits of light therapy — but the type and timing of antihistamine can screw things up if you’re not careful.

Light therapy works by hitting your eyes with bright light in the morning, which shifts your circadian clock and boosts serotonin. The problem is that some antihistamines — especially the older ones like diphenhydramine (