Many people can hear their heartbeat. This is phenomenon known as pulsatile tinnitus. Pulsatile tinnitus may be more prominent at night, when you’re lying in bed, because more blood is reaching your head, and there are fewer external sounds to mask the tinnitus.
Pulsatile tinnitus is usually related to blood flow, either through normal or abnormal blood vessels near the ear. Causes of pulsatile tinnitus include significant dehydration, anemia (lack of blood cells), pregnancy, overactive thyroid gland, or benign growths in the blood vessels near the ear. There are also some medications that may induce it.
If you notice any new pulsatile tinnitus that persists for a few weeks, it is better to consult an ear-nose-throat specialist on the matter.