Where do you live? Are there actual spiders in your area? In my experience, a lot of people attribute painful skin infections, abscesses, boils, etc. to spider bites when in fact spider bites are quite rare. Also, they typically have a central painful black area called an eschar with a surrounding area of redness.
Fleas absolutely bite people, usually at night, and usually in exposed areas like the ankles and wrists. You might actually see/catch fleas, but they are very small and quick. Other things that can bite at night include mites, mosquitoes and the dreaded bed bugs.