Exposure lights can have a battery pack (fuel cell?) that mounts on the back of your helmet too. I think a Joystick would then run for 48 hrs. Both the light and/or the fuel cell can be used to charge devices/phone etc. There's a reason (or two) that they are more expensive. As above though, it's not often a light is needed for long, unless you're in it for the win.
On my only HT experience, I had to camp in the rain at 2am in Glencoe because the cheap Chinese head torch I tried to get away with failed - though that was a blessing in disguise....
I have vivid memories of following the rider ahead of me (@touch) over the Devil's Staircase, thinking that if I could just catch up, I could follow him on the descent - as the moonlight was only good enough for climbing slowly. The replay on Trackleaders has him a few of hours ahead of me. Either someone was out at midnight for a nightride, or I was hallucinating after a long day
@touch - Did you go from Dundonnel (mile 350) to the finish, without any sleep that year (2015)?