I'm confused on why the value of an hour isn't just your hourly wage. If you value your time more, you'd be working less. If you value your time less, you'd be working more. Since you clearly spend exactly as much time working as you do, you value your free time that much.

Here's a secondary justification beyond the economics substitution arguments of the others.

Assuming you have a job at all which could have an hourly wage, the wage must be equal or greater than the minimum wage. The more your hour is worth, the more valuable melatonin use is. The most pessimistic or conservative assumption is assume the hour is worth as little as possible, and the minimum wage is the least it's worth.

Hence, using the minimum wage and not one's actual hourly wage is the most pessimistic assumption - but the analysis still says the benefits are positive.