Last August 14, I did a one-day hike to the summit on the main Whitney Trail leaving at 12:30am on a full moon evening/morning. I did use a headlamp, but mostly for the water crossings. I had a very clear morning hike and, with the illumintaion from the moon, the headlamp was almost unnecessary. However, most folks seem to cover the round-trip distance much faster than I did (I arrived back at the Whitney Portal at 9:00pm!) and I was very tired by the time I got back to Lone Pine Lake. From LPL to the trailhead, I used my headlamp constantly...even though the batteries were beginning to fade.....Bring an extra set of batteries. By the way....I had a great time doing a "Midnight Hike" and will do it again.