"How have women historically survived the pain of childbirth without the drugs? They had no choice, but luckily, nature prepared women's bodies for such an event.
When the brain perceives pain (especially with the added stimulus of stress) endorphins are released. Endorphins are chemical compounds secreted by the brain and adrenal glands, and have a pain-relieving effect ten times more potent than morphine. They are also mood elevators (e.g. "runner's high").
As dilation progresses the sensation of pain will increase. The more pain you have, the more endorphins are released to help you cope. The rising level of endorphins contributes to the shift from a thinking, rational mindset to a more primitive and instinctive one. Endorphins take you to the dream-like state of Laborland, which meshes well with the tasks of birthing."Endorphins also make your memory a little hazy ... which is also quite convenient, as labor is quite intense and women might have historically fought men off if they could remember every detail.
An interesting side note to all that: drugs stop the creation of endorphins. Your body is no longer under stress, and so it does not create your own pain killer any more. So if you take some sort of drug to rest for a while, when you come out of it the pain will be worse. So as you are progressing and things get rough, see where you are at. The pain of contractions usually peak around 7 or 8 centimeters, so if you are about to that point, try to keep going. See if you can make it over that hill. But if it has been an incredibly long time and you are beyond the point of complete and utter exhaustion, take a break. Maybe take something to numb you up for a while.