"I have trouble waking up early..."
And this is a problem why?
I usually go to bed when it's dark, and get up when it's light. Fortunately, that works around here - but I could see it not working so well if there were mountains or tall trees that delayed sunrise for you. Probably the lightest choice would be some sort of alarm built into a watch or other electronic item you already carry. (Do GPS or PLBs have such a function?) I've also seen some carabiner-type thingies that have watches, alarms, compasses, etc. all built in; you might browse at these next time you're in an outdoor shop. I'd want to find out how easy they were to set, and how loud they ring, before buying one.
If you're a sound sleeper, you might try pinning it to your pillow, or putting it in a shoe (my shoes usually end up under my head, as part of my pillow) or otherwise placing it near your ear.