Myth: "A spider bit me while I was asleep. (No, I didn't see any spider, but what else could it have been?)"
Fact: The notion that "if you didn't see what bit you, it was a spider" is (to me) one of the strangest of the widespread spider superstitions, already well established in 1901 according to a medical article published then. Even some physicians, who really should know better, accept it! I have no idea how this belief originated, but it is quite false.
Here are some facts: Unless you are sleeping on the basement floor, a spider might wander onto your bed as often as twice a year. Not every night! If you take elementary precautions like not letting the blankets or bedspread touch the floor or walls, the incidence of spiders on the bed will be effectively zero. If a spider does get on a bed, usually no bite will result. Spiders have no reason to bite humans; they are not bloodsuckers, and are not aware of our existence in any case.
If you roll over onto a spider, most likely the spider will have no chance to bite. Being crushed against a bedsheet by a human body just doesn't work well as a biting scenario (despite what everyone thinks) because spider fangs are underneath the spider. When pressed on from above, the spider may reflexively bite what it is standing on: the sheet, not your body.
True spider bites (which are rare events) occur when a spider is trapped inside clothing or when someone foolishly puts a hand or other body part in a spider habitat without looking, or even more foolishly slaps at a spider that is crawling on them.
Skin bumps and sores noticed in the morning are generally caused by non-bite disease conditions: see this article for a partial list. Currently MRSA bacteria (see this article and this one) are among the leading causes of alleged "spider bites." The minority that are really bites are caused by bloodsucking insects such as fleas, bedbugs, kissing bugs, lice, or assorted flies; less commonly by mites or ticks. Genuine spider bites in this situation are possible, but very rare.
Information on this web site is not a substitute for professional medical advice, and should not be used to diagnose or treat a medical or health condition. You should consult a physician as to any symptoms that may require diagnosis or treatment. Genuine spider bites can sometimes require medical attention, but beyond that, several medical conditions commonly mistaken for spider bite can be even more serious. If you have what appears to be a serious spider bite, please contact your health care provider or local emergency services. If you have the actual spider that bit someone, always save it for identification by a professional arachnologist.