Graph showing the pressure to sleep. The y axis shows the pressure to sleep ranging from low to high. The x axis shows time periods labeled awake and asleep. A shaded area is shown above the word “Asleep” from the bottom to the top of the scale. A solid line is shown beginning at the left next to Low on the vertical scale, curving down slightly to the baseline, then curving two-thirds up the vertical scale to the shaded area above the word “Asleep” where it changes to a dashed line, representing staying awake, and continues upward past the shaded area. Where the solid line changes to dashed, another solid line curves down into the shaded area above “Asleep” to the baseline and then begins a slight curve up past the shaded area.