This is an excellent summary from Dr. Dan Siegel, clinical professor of psychiatry at the UCLA School of Medicine, which explains why bedtime screen viewing can have a negative effect on the brain and body:

https://www.businessinsider.com.au/smartphone-impact-brain-body-sleep-2015-2?utm_content=buffer37baf&utm_medium=social&utm_source=linkedin.com&utm_campaign=buffer