Scientists are not sure about how LSD works, but the theory is this:

LSD stops cells from giving off a neurotransmiter, (serotonerhic), which alters activity in the brain. Some neurons are temporarily disables without this neurotransmiter, while others are stimulated without it. Serotonerhic is thought to be involved in the control of sensation, sleep, attention, and mood and this is thought to be why LSD alters these systems.