Guardian feature writer Rhiannon Lucy Cosslett recently wrote about the growing cohort of parents whose children are on the waiting list for an autism or ADHD diagnosis, and are turning to the internet to buy melatonin to help them sleep. She tells Madeleine Finlay about their experiences and what is driving them to the hidden market. Paul Gringras, a consultant in paediatric sleep medicine and neurodisability, and lead of sleep medicine at King’s College London, also explains why melatonin can be helpful for neurodivergent children and why he is concerned about the increasing number of parents looking for it online