When loud snoring is interrupted by frequent episodes of totally obstructed breathing, it is known as obstructive sleep apnea. Serious episodes last more than ten seconds each and occur more than seven times per hour. Apnea patients may experience 30 to 300 such events per night. These episodes can reduce blood oxygen levels, causing the heart to pump harder. The immediate effect of sleep apnea is that the snorer must sleep lightly and keep his muscles tense in order to keep airflow to the lungs. Because the snorer does not get a good rest, he may be sleepy during the day, which impairs job performance and makes him a hazardous driver or equipment operator. After many years with this disorder, elevated blood pressure and heart enlargement may occur.