"People who are contented and serene sleep well. They fall asleep easily, stay asleep, and wake refreshed. Conversely, people who are anxious, stressed, or depressed do not sleep well, and chronic insomnia is strongly associated with mood disorders. These are clear correlations, but what is cause and what is effect is not clear. Most experts agree that sleep and mood are closely related, that healthy sleep can enhance emotional well-being, while insufficient quantity or quality of sleep can adversely affect it."