It sounds like your daughter may have some anxiety happening. Sometimes this occurs as a result of shifting hormones, sometimes because of traumatic experiences and sometimes because of an anxious nature (a genetic predisposition). A mental health professional can help figure out the source of her fear. If you want to try something simple before taking her for professional assessment and treatment, I'd recommend a round of Bach Flower Therapy. Consult a Bach Flower Practitioner or go to your local health food store and purchase the following flower essences: mimulus, aspen and rock rose. Put 2 drops of each of these in a Bach mixing bottle (sold where the essences are sold) filled with water and a teaspoon of brandy (to preserve the mixture). Put 4 drops of the mixture into a small amount of liquid (8 oz or less) such as water, juice, chocolate milk, soda, breakfast cereal etc. The preparation will be tasteless and doesn't interact with any food, medicine, health condition or allergy that she might have. She will need a sip of the Bach-liquid 4 times a day (morning, mid day, early evening, bedtime) until she is no longer fearful of sleeping alone. Check out www.bachflowers.com for more information. If the treatment doesn't work, it won't have harmed her in any way (except by prolonging the problem for a few more weeks). If after a few weeks she is still fearful, I would recommend a consult with a professional.
Sarah Chana