Overview of ADHD Sleep Disorders
Some children who have been diagnosed with attention deficit hyperactivity disorder also find themselves dealing with sleep disorders. That is, as they get older, they find it more difficult to drift into healthy, restful sleep at night. Explore several non-medication and medication methods to help your child reverse ADHD sleep disorders.
“Sleep hygiene” combines everything that helps the child with ADHD to fall asleep and stay asleep in a healthy pattern every night. What works for one child may not work as effectively for another child, so you need to find out what tactics are the most effective for you, your child and your family. 
Some techniques include complete quiet in the room, or having a source of “white noise,” such as a fan, taking a hot bath about an hour before bedtime or avoiding food before going to bed. You might even think about getting the rest of your family to use some of these practices as well, so that your child with ADHD will not feel “targeted” for special treatment. They will also enjoy the benefits of healthy bedtime routines.
Bed is Only for Sleep
Teach your child that the bed is only for sleep. Do not allow activities like doing homework, watching TV, lounging, playing games, reading or visiting with friends to take place on the bed. 
Your aim is to teach your child with ADHD to associate the bed with sleep and nothing else.
Caffeine in foods and drinks (chocolate, sodas, tea and coffee) mimics the stress response.  In your child with ADHD, this response can spell sleepless nights.
Go through your kitchen pantry, cabinets, refrigerator and freezer and get rid of any food items containing caffeine. Make sure that beverages and foods you give to your child with ADHD are caffeine-free; if you and your spouse enjoy drinking tea or coffee, make sure your children understands that only the adults in the home are allowed to drink them. If your child with ADHD enjoys tea, buy decaffeinated herbal tea.
Consistent Nighttime Routine
Look at the routines in your home. Children with ADHD need strong structure and routine in their daily lives. This extends from meals to school and homework to nighttime routines. 
Make sure your child with ADHD follows consistent routines every day – ensure that bedtimes and waking times are the same every day, even on weekends and during school holidays.
ADHD Stimulant Medication
ADHD can coexist with a sleep disorder, according to the National Institute of Mental Health.  If your child with ADHD finds it difficult to settle racing thoughts which prevents a good night’s sleep, a discussion with the doctor (pediatrician or prescribing psychologist/psychiatrist) may be in order.
Because of the “paradoxical effect” an ADHD stimulant medication has on a child with ADHD, taking a dose of medication about 45 minutes before bedtime can help.  The paradoxical effect is where it is believed that stimulants are able to calm those with ADHD, and energize those without.
Check for Medical Problems
Your child with ADHD may suffer from other medical problems that interfere with healthy sleep. These conditions can include sleep apnea, allergies or asthma.
Monitor your child’s sleep – any symptoms like restless sleep, snoring or breathing disturbances need to be reported to your doctor so appropriate treatment can start. 
Children with ADHD feel the need to move, sometimes incessantly. Encourage your child to release some of that excess energy with physical exercise. If the weather is pleasant, send your child outside, where the combination of fresh air and physical exertion might allow your child to feel sleepier at bedtime.
Make sure that, when your child engages in physical play, that it is not right before bedtime because of its stimulating effects. 
Melatonin is a naturally occurring peptide released by your brain. This peptide helps to set your circadian clock, which helps you set your daily sleep and wake cycles.
If your child suffers from ADHD sleep disorders, this is a natural remedy to explore and think about. Because melatonin is a natural substance, it may take several nights before your child experiences any benefits. The most beneficial dose is 1 mg or less. 
Give your child a hot bath about one hour before bedtime. Sleepiness usually follows the body’s cooling after a bath – this is another natural, non-drug strategy to try.  In addition, try to keep the temperature of your child’s room on the cool side. Doing so may help to influence your child’s sleep cycle.
https://www.additudemag.com/adhd/article/757.html ADDitude: ADHD Sleep Problems: Causes and Tips to Rest Better Tonight!
https://www.nimh.nih.gov/health/publications/attention-deficit-hyperactivity-disorder/complete-index.shtml National Institute of Mental Health: Attention Deficit Hyperactivity Disorder (ADHD)
https://www.webmd.com/add-adhd/guide/adhd-sleep-disorders WebMD: ADHD and Sleep Disorders