Many causes. There are many types and presentations of sleep disorders, ranging from inability to sleep (insomnia) to disrupted sleep to too much sleep. Excessive worries, medical problems, activity levels and day-night patterns, sleep apnea (sleep breathing problems) are among the common causes. When there is sustained sleep disturbance, daytime function is affected, resulting in fatigue, memory trouble, etc.
Sleep disorders. Could be weight related.
Habits, light, genes. Delayed sleep phase syndrome is a partly genetic condition whereby one's sleep-wake schedule is greatly delayed with respect to the timing of one's internal biological clock. Causes include 1) habitually intentionally staying up late (e.g. To play video games), 2) exposure to too much light before bedtime, 3) exposure to too little light upon awakening, 4) genetic predisposition, 5) certain meds. Read more...