Is it a surprise that both play a part? Probably not. However, there is a difference between night-time sleep and napping. According to this study of 500 sets of identical and fraternal twins, naps were most influenced by environmental factors. http://news.yahoo.com/kids-genes-environment-affect-sleep-differently-041243139.html This was found to be especially true as children got older. This points to a fact that sleep consultants are well aware of, napping is the most challenging aspect of child sleep training. In order for a child to get the restorative daytime sleep (s)he needs, the environment must be dark and quiet, and the time of sleep needs to fit with the child’s biological sleep clock.
This study also found that at the age of 18 months, a child’s sleep environment did affect night-time sleep as well as daytime sleep. Why this happens is unclear, but this often is a time when parents enlist the help of a sleep consultant to ensure their child’s sleep needs are being met.