More often than not, "fast and light" is the best way to climb a mountain: pack a bag with the minimum amount of gear you think you'll need and then go as fast as you can until the job is done. Generally, the light bit is pretty straightforward once you've done enough mountaineering to accurately judge it, but going fast is tricky. As a result, you might find yourself in the unfortunate position of having gone light, but not fast, and be forced to spend an uncomfortable night on a mountainside. The problem with seeking your limit is that one day you might find it, and the price you pay for this self-discovery is a whole night of shivering and playing "guess the time" with your climbing partner. (In my experience of "benightments"—the climber's term for an unplanned night in the mountains—less time has usually passed than you think/wish, so guess early to maximize your chances of winning the time game.)
How light is too light on a mountain?
So - exactly how light is too light? The adage goes that if you take a bivouac kit, you'll use it, but taking no emergency gear is a dangerous business. My approach when trying to push the limit of what I can do in one day is to take no proper bivouac gear but some tiny, lightweight shelter or insulation. Even a foil blanket will make a massive difference to your warmth if you do get stuck out, and they weigh virtually nothing. A super thin bivouac sack or group shelter will also be a game-changer on a cold night, so I think the extra few hundred grams is worth carrying.
At the very least, taking a slightly warmer jacket than you're expecting to need is a good idea. Three hundred extra grams is unlikely to be the difference between doing a route and not, but it could be the difference between a bad night and a terrible one!
What to do when you get stuck on a mountain
It's a funny feeling when you realize that you're in danger of being stuck out unexpectedly. There's fear initially as you start doing calculations about distance vs. daylight and realize that it's tight. Still, as the possible becomes probable and then certain, I've personally found that the stress washes away somewhat, and I rather calmly accept my fate. Once you know that you've got a long and cold night ahead, you have little choice but to toughen up and get through the ordeal to come, and that lack of choice in the matter is - for me at least - easier to deal with than uncertainty. As Mark Twight put it in his book Refuge, "Bend with what comes until bending is no longer the right response. And then be as hard as hard can be."
When you know it will happen, stop pushing on as far as you can with the daylight you have left and instead start looking for the best spot to spend the night. If you can get a bit of sleep, you'll be functioning far better the next morning, so giving up an hour of moving time because you've found a flat and sheltered ledge will be more than repaid the next day when you can operate effectively. Pushing on right until you can go no further usually results in you spending the night in an uncomfortable spot, limiting your ability to sleep and leaving you destroyed the next day. Time spent making the night as comfortable as possible is better spent than climbing another pitch or two in the last moments of daylight.
In terms of tactics, the goal when benighted is to limit the amount of cold that can get in, be it from the air or the ground. As such, insulating yourself from the mountain is vital - sitting on your pack and neatly laying out the rope to provide as much cover from the ground as possible is step number 1. Carrying things like buffs is also a good idea because they weigh virtually nothing but allow you to limit the amount of cold air coming in through the top of your jacket, and they can also be converted into makeshift hats to supplement whatever headwear you already have.
One thing that will make a huge difference to your warmth and ability to sleep is having food in your belly - doubly so if it's rich in fat. On one benightment I had some chocolate left over from the day and some pepperoni meat sticks, and I found that when I ate some chocolate I could doze off for 10 minutes before waking up shivering, but I could sleep for up to an hour after eating a few bites of pepperoni. Fatty foods keep you warmer than sugary stuff, so if you think there's even a slim chance of an unplanned night out, putting an extra bit of fatty food in your pack could end up paying you back in a big way.
Speaking of sleep, it's important to try and get some. You will likely be tired from the day's exertions, so if you can get some calories inside you and have enough kit to not be too cold, you should be able to get some shut-eye. That said, it's also important not to just hunker down for the night because frostbite could be an issue if it's really cold. Eating, trying to sleep, getting up, and stomping around for a few minutes is my recommended routine.
And when the sun returns?
When the morning does finally arrive, don't rush. If you initially thought you could do your route in one day you're almost certainly going to be back to safety without another night in the open, so let the sun hit you and soak in the warmth - it makes getting moving much easier than trying to do it in the frigid cold.
The final task with a benightment comes in the days and weeks afterwards, as you analyze what happened. Did you take too much gear and go too slowly, or should you have taken more climbing hardware, which in turn would have made placing protection quicker, easier, and faster? There are usually some good lessons to be learned from a benightment, so do your best to figure them out.
At the very least, getting benighted provides you with a pretty cool story to tell and a newfound appreciation for your bed!
And check out these routes if I haven’t put you off (!) and you want to begin learning the mountaineering game: