Sleep backlight settings take precedence over activity backlight settings

I noticed yesterday that my display didn't light up while working out after sunset, even thought gesture mode was turned on for activities. It took me until later that night that I realized that my activity took place during my sleep window. It still seems a bit odd that the sleep settings take precedence over the activity settings, because there is no way I'm sleeping while doing an activity. Is this considered to be a bug, or a feature?