Update your Caffeine to Amphetamine (Mac app)
Keep your Mac awake during a presentation. Or whenever your Mac needs to stay awake. The free Caffeine used to do this well, but the times they are a changing. And we need something that works better with modern Mac OSes. Enter Amphetamine. Better in every way to keep your Mac awake.
I switched because of the small visual issues with Caffeine El Capitan and stayed just because of the custom time (handy during presentations/training sessions).