Kilimanjaro - Midnight at Barranco
The comments on this page are written with climbers in East Africa in mind and in particular those tackling Kilimanjaro. However they apply equally well to ascents of other major mountains such as Elbrus.
Almost all nights in the East African mountains (excluding the Rwenzori) are clear and for many mountains such as Kilimanjaro, Mount Kenya, Lengai and Mount Meru a pre-dawn start is crucial. To make the night ascents more pleasant and in some cases beautiful, getting up and walking up by moonlight is the optimal situation. As a rough guide a full moon rises at sunset and sets at sunrise which is just about sunrise. This combined with strength of the moonlight provide excellent conditions for walking at night. However as the moon sets about 1 hour later every day. the best conditions are probably about 2 to 3 days after the full moon as then at about 5am the moon is still big and quite high in the sky throughout the critical hours of the scent. If you plan your ascent before the full moon you might get up by moonlight but it will set before twilight. An ascent a week after a full moon means you will have a half moon rising about midnight and this will be overhead in the sky around sunrise. If you have some flexibility in your plans make use of the moon!
Why get up in the middle of the night anyway? An early edition of the MCK guidebook summed up the situation quite well (referring to the final ascent day on Kilimanjaro) - you feel sick and you cannot sleep so you may as well get up anyway. then when you start it is cooler. the scree is frozen so it is more stable and you cannot see where you are and how far you have to go so you do not get so depressed. This maybe a bit exhaggerated but is not far off the truth. Good acclimatisation will help a lot reduce discomfort but another very important factor is the sun during the day, can have a very debilitating effect on the body. On Kilimanjaro the air temperature may not be very high but the high levels of radiation can completely drain you of energy. On mountains such as Lengai it is almost dangerous to start the ascent too late as the day time temperature soar and heat exhaustion could at least force you to retreat and in extreme circumstances heat stroke can be fatal.
A final advantage of a night start is that you have a longer day available to you - being caught out by nightfall at the end of a hard day, particularly if the weather has deteriorated during the day can be very unpleasant. Early starts add to your safety margins.