Left to their own devices, your eyes can adapt to the darkness by producing a photosensitive chemical called rhodopsin.  But once you turn on the torch the rhodopsin breaks down and your built-up night vision is lost.  That’s why a headlamp with a red light setting is strongly recommended for night time activities.  Red light is enough to read a map, but won’t leave you blinded. The Ramblers Association has a list of head torches of all shapes and sizes, like the recommended Petzl e+LITE pictured above.