The results of a facebook survey indicate that 98% of all geocachers have at some point cached at night. Sometimes that’s on purpose, they are looking for a night cache and other times the desire for one more find outweighed the need for daylight. Whatever your reason for being in the woods at night you should considering leaving a flashlight in your pack at all times. A small headlamp that uses AA batteries is a good choice if you intend on being out at night. Most GPS units use AA batteries so your spare batteries will work in your AA flashlight.
Here is an important tip if you keep a flashlight in your pack but rarely use it. Consider using either alkaline batteries or low self-discharge rechargeable batteries such as the eneloop brand. Typical NiMH rechargeable batteries lose 1% of their storage capacity per day of storage. They are almost unusable after 30 days.