I've been doing HIIT 3 times/week on the bike on top of doing a 5/3/1 routine four times/week. Right now I'm doing 12 minutes of HIIT alternating 30 seconds rest with 30 seconds sprint and I've been increasing this total time by a minute each week with my goal to be 15 minutes at a go (which I'll reach in a couple more weeks). I guess what I want to know is how should I go about changing my HIIT routine up instead of just continually increasing the time. Shorter rest periods maybe? My overall goal is to lose fat slowly.