A typical U.S. adult gets 6.8 hours of sleep per night. A sleep researcher tested if college students sleep less than the general population. He asked 24 college students how much sleep they got the night before (in hours) and found that on average they slept 6.3 hours. The researcher is running a one-sample, one-tailed t-test to test his hypothesis. How many degrees of freedom are there for this test?