Yes ... it is dark at night. The classic way is to start over Crib Goch about six in the morning, drop back down after Snowdon summit from the midway rail station for breakfast, cross the Glyders and Tryfan for lunch in the big parking area in Ogwen, then refreshed, plod up Penyr Ole Wen (the worst bit, you are getting tired) then a stroll in the evening sunshine along the Carneddau ridge. If you are thinking of 24 hours, forget it. You will simply get too tired for the time you have spent on your feet. Fifteen hours is very reasonable, just walking at a steady pace. A great experience. The secret is not to rush. Just keep walking at your normal pace. Let us all know.