Complex sentence: Increased levels of melatonin causes a downregulation of leptin, however, melatonin also appears to increase leptin levels in the presence of insulin, therefore causing a decrease in appetite during sleeping.
Split into simpler sentences: Increased levels of melatonin causes a downregulation of leptin. However melatonin also appears to increase leptin levels in the presence of insulin, therefore causing a decrease in appetite during sleeping.