For dieting? Change to another med is one solution. In the meantime use excellent sleep hygiene. 1. Keep a regular bedtime & wakening time (to include weekends). Get up the same time each day, regardless of what time you fell asleep. 2. Make a regular, relaxing bedtime routine. Relaxing rituals prior to bedtime may include a warm bath or shower, aroma therapy, reading, or listening to gentle music.
Stop the drug. If you are taking an amphetamine rx'd by a dr. I'd assume they dx'd adhd. If you are experiencing insomnia i'd question the dx (diagnosis) or you may be taking a dose too late in the day.