On night watch, you will inevitably have to trim a headsail in total darkness. The moon has either not risen or is new, offering no illumination on the matter at hand. Stars, while bright and beautiful, will not grant you vision of your headsails either.
You could shine a flashlight at the sail, but this will only destroy your night vision making the remainder of the process even harder.
So how do you trim a headsail in the dark? By feel.
If the headsail is too eased, it will flutter and that fluttering can be felt in the sheet as it approaches the winch. Simply ease the sail until you feel it start to flutter, then winch it in until this damaging vibration ceases. At this point, the sail is trimmed in as little as possible so you don't risk being over-trimmed either.
Next time you can't see your sail, simply touch the sheet and let your fingers see for you.