These drinks are not FDA approved and are labeled as dietary supplements. I personally tried to find some of these drinks on the shelves of my local supermarket and gasoline station to see what they are about.
Furthermore, Marley's mellow mood did include an ingredient list: a relaxation blend of 80mg per bottle, and listed chamomile flower extract, lemon balm leaf extract, valerian root extract, hops extract, and passionflower extract. However, the specific amounts of each component was not provided.
The one ingredient that stood out was the varlarian root extract. Valarian root is one example of a dietary supplement. It may help with anxiety or insomnia, however there are limited clinical trials on valarian root (or other dietary supplements for that matter) to begin with, and trials that looked at valarian root used doses of at least 150mg. So the amounts in these relaxation drinks like in the "Marleys' Mellow Mood" has questionable efficacy.
I came across an article on consumer reports which goes hand-in-hand with what I found. There is a huge variety of these type of drinks on the shelves and most of them contain unspecified amounts of melatonin, valerian, L-theanine, and others.
In short, drinking one of these occasionally is probably fine for most healthy adults, but make sure to read the labels for warnings and maximum daily servings. It is especially important to take caution if one is already taking dietary supplements. For some more information, please refer to the sources below.
Posted by: Adrian Lee, WesternU PharmD Candidate 2014