Before treating insomnia, you should first determine the real cause for it. Insomnia, or difficulty falling asleep, can be due to either a mental or organic disease. Treating the primary disorder can help a person overcome the case of insomnia. In your case, your primary mental disorder (ADHD) could cause insomnia. In addition, some medications can have a hypnotic effect and act directly on insomnia. There are several groups of medications used to treat insomnia:
1. Benzodiazepines: temazepam, diazepam, lorazepam, flurazepam, nitrazepam, midazolam etc.;
2. Nonbenzodiazepines: zolpidem and zopiclone;
3. Antidepressants such as mirtazapine, trazodone and doxepin;
4. Antihistamines: hydroxyzine, diphenhydramine…;
5. Atypical antipsychotics: quetiapine (Seroquel);
6. Melatonin and its agonists including Ramelteon (Rozerem)
7. Herbs such as valerian, chamomile, lavender, hops, and passion-flower, cannabis…

Other treatment methods include:
1. Cognitive behavior therapy;
2. Aromatherapy;
3. Relaxation techniques such as meditation;
4. Traditional Chinese medicine etc.

You can consult a mental health doctor about your primary mental condition and for guidance in treating the current case of insomnia.