Between hectic work schedules and full social calendars getting a full night's sleep is no easy feat these days. Sleep disturbances (e.g. not being able to fall or stay asleep) are especially common among people who have mental health disorders. And with the immense desire to get that precious rest, many of us also build up a separate kind of sleep anxiety (technical term: psychophysiological insomnia).
For example, it is not uncommon for your wish to sleep to become its own source of anxiety. Some start tensing up just knowing that bedtime is approaching. And then there's the particular frustration of waking up during the night, only to become anxious and unable to fall back asleep simply because you're freaking over how tired you're going to be the next day.
The point: Sleep, as crucial and wonderful as it is when you can get it, can be a nightmare for anxiety-prone people sometimes.