During sleep, a person's heart rate generally decreases and becomes more regular. This is because the body's muscles and organs relax and the body's metabolic rate slows down, which leads to a decrease in heart rate.
However, there are certain sleep disorders, such as sleep apnea, that can cause a person's heart rate to increase during sleep. Sleep apnea is a condition in which a person's breathing is repeatedly interrupted during sleep, which can cause their heart rate to increase and their blood pressure to rise. In general, if a person's heart rate increases during sleep, it may be a sign of a sleep disorder or other underlying health condition, and it is important to speak to a doctor for further evaluation and treatment.