Hey Melissa, we are programmed to wake up naturally after each REM cycle, so technically you are not interrupting the sleep cycle. However, I wouldn't recommend waking up and writing down all your dreams after every cycle. I don't think that's very practical and may be a bit invasive especially if you haven't been doing this all along. For starters, I would say make it a habit to write down your dreams after your final wake. See how much you can recall. Then, the next step will be to try to notice your natural wakings. Take some notes about these so that we can analyse the situation and determine what to do from there.

Regarding the light, it is obviously necessary unless you find an alternative technique such as mental journalling that does not require writing.