I don't have a particular reason other than time, but I can tell my night vision isn't what it once was, especially driving on rainy nights with oncoming traffic. Yuck.

Having done some research, I found you should wear the darkest sunglasses possible, as being in sunlight affects your dark vision that night. (Pilots who will be night-flying, sailors on night watch, etc. have guidelines about this.) Chemical changes occur in the eye when exposed to light, and our color vision is most accute in the yellow-green portion of the spectrum (think green fire trucks). Even a minute with a flashlight can affect night vision the next hour or two. I try to make my nighttime pitstops without a light to avoid that--the stars are usually worth watching during the unwanted trip outside my bag.

Programmable headlamps with discrete red mode and a low white mode you set yourself are my preference. A switchable spot/flood beam is likewise helpful and I want a light that switches on to the last mode used--I close my eyes when switching on common lights that always start on highest setting (I swear they do that to make them impressive in the store.)

I use the Petzl Tixxa XP with Li-ion Core battery (now discontinued). The battery has regulation circuitry and the white high/low settings are programmable (using a PC at home). It also has a diffuser lens that turns the spot beam into a flood. When walking after dark I start with no light, then red, then white low and usually only call on the high setting when scouting into the distance or navigating a tricky bit.

And on that last, the ability to hike at night is hugely impacted by the terrain and quality of the trail. A typically rocky Sierra path is often indistinct and criss-crossed with game and horse trails, and adds the thrill of a potential rolled ankle or worse. If it's a 1.5 mph trail during the day, it's at best 0.5 mph at night (plan accordingly). OTOH a clear tread across an alpine meadow usually won't require any added light whatsoever.