I just got home from working my first double. I started with UPS in January on midnight unload. Yesterday I started at 10:15pm. At 2:00am I went over to hazmat to rebox and browntape untill 3:30am. On a tip from a co-worker I walked back to my sector and talked to the sunrise supervisor. He threw me into a couple of longs until 6:00AM. A co-worker on sunrise told me not to do doubles too often because it is draining. I feel tired but nowhere near as tired as when I first started. Planing on doing this 2 or more nights a week for a few weeks and then gauging how my body feels about it.
I think the key to get some more hours would be to talk directly with the supes incharge of the shifts you are interested in. You might have to check in with them nightly to see if they need some help, but it may be worth it in your case. Often they don't know until start who won't show up or even what the volume will be.
I should mention I work in a huge hub (250+ doors), so it may be harder for you in a smaller building. GL.