I'm not very good at telling when I need more sunlight, exercise, or even food. But I can tell when our pet needs things, and find it pretty motivating to take care of them (and hence myself).
At one point, years ago when I was so depressed I could barely get out of bed, I realized "If someone treated a dog the way I've been treating myself, I would take steps to get that dog taken away from them." Food every day, water several times a day, walks, baths, interaction, play.. I was doing none of these things. It really helped to try to think of myself as an "other" that needed care.