Graph depicting the biological clock. The y axis shows days ranging from 1 at the top to 45 at the bottom. The x axis shows two midnight to noon to midnight periods, with a vertical line separating the two periods. The top portion of the graph is marked with a horizontal line at 10-days on the vertical scale and labeled entrained to 24-hour day. Another horizontal line is marked at 35 days, and the middle portion of the graph between 10 and 35 days is labeled absence of environmental cues. The bottom portion of the graph from 35 to 45 days is labeled entrained to 24-hour day. Horizontal bars span 24-hour cycles. Dark portions of the horizontal bars represent time asleep, and light portions represent time awake. For all horizontal bars, the dark portion is approximately one-third and the light portion two-thirds of the total length. A bar is shown for each of 45 days. Beginning at the top of the graph with day 1, the bars span the same 24-hour period for 10 days; then gradually shift to the right for days 11 through 34, spanning parts of both days; then span the second 24-hour period for days 35 through 45.