阅读下面材料，在空白处填入适当的内容（1个单词）或括号内单词的正确形式。

    Happiness means different things to different people. For Mary, who has ____ severe sleeping problem, happiness is as simple as having a good night's sleep.

    Bed was once a place ____ Mary went to rest after a long, hard day. ____, she has not had a full night's sleep in almost six months. Her doctor said that ____ (count) sheep might help her fall asleep. She tried this, but soon got tired of it after it failed to work. She finds the nights without sleep almost ____ (bear) . She is always sleepy and lacks energy during the day.

    Mary is not alone. Studies show that more than one in three people worldwide struggle with getting enough sleep. It often takes on different ____ (form) : some people can get to sleep, but wake up much too early; some get a full night's sleep, but are ____ (disappoint)  to find that they still feel tired when they wake up; others, like Mary, find ____ difficult to fall asleep in the first place.

    Specialists say that people should not worry too much about this problem. In most cases , a lack of sleep ____ (cause) by anxiety and pressure. It is important to keep yourself ____ good spirits and to be positive.