Can you see bed bugs?

When I go to bed at night I have extremely itchy skin, particularly on my back. Sometimes I lay awake for hours because of it. I also discover spots of blood but that’s most certainly where I’ve been scratching. It also itches during the day especially if I get hot, but less so. I’ve tried searching my mattress for signs of bed bugs but can never see any. If I can be certain that’s the cause I’d buy a new mattress and don’t want to spend money on one only to discover nothing’s changed. Has anybody experienced this problem and are bed bugs visible to the human eye?