when i took my son to florida we gave him anti hay fever type medicine(for kids) from bootsbecause one of the side effects was drowsyness and he slept for about five hours and was right as rain after.For good advice see a Boots pharmacy person
When we go to the states we always buy Melatonin from a local chemists, and take one before bedtime. It is a herbal supplement that makes your brain release the "sleep chemicals" so your body has a natural sleep at the time you want