Most spiders will not actively seek a human to bite. In fact, most spiders will turn tail and run if they are able too. The reason nighttime spider bites are fairly common is because a sleeping body (humans tend to roll while sleeping) has rolled on top of them.
Spiders will, however, actively seek warmth (especially in cooler climates) aka next to a sleeping person. With an average body temperature of 98 degrees, we do make rather effective heaters. Combine this with the first point and you find that spiders seeking warmth are sometimes rolled on top of, and they bite as a response.
Spiders do not bite you in your sleep. Strange marks do not mean spiders. There is a LONG list of ailments that can cause strange marks on you, from bedbugs to an infected mosquito bite to flesh eating bacteria.
Will is correct and pretty much covered what I was going to say.
Spiders generally try to avoid humans (think about how much bigger humans are than spiders), but, like many other small creatures (like mice), they will bite if they feel threatened. Like Will said, they are attracted to heat, and may be trying to stay near the heat source (especially the part closest to the blanket, as that traps the heat), but when it rolls over, it may think that the human is trying to kill it (once again, the human is much bigger than the spider) for whatever reason.
By the way, there is a chance those bites could be bedbug bites, not spider bites.
if it was a spider bite you'd see the two fang marks on your skin. if you just see a bump its most likely a staph infection.
if you put a cold blooded creature who found a nice warm spot and you put your hand down on it and its stuck it'll bite the heck out of you to get away.
most of the time people have a staph infection and claim a spider bite them
also when a spider bites you its like a bee sting it hurts alot enough to wake you from a deep sleep
so if you see a large itchy lump its a staph infection