Melissa officinalis (Lemon Balm) is a herb that has traditionally been used for a variety of cognitive purposes, most of which are centered around improving cognition and reducing stress and anxiety. It is said to calm the nerves and to relax the body.
In regards to its subjective cognitive effects, it appears to be effective in inducing calmness. This may also manifest itself in a negative manner as healthy persons who take Lemon Balm prior to a cognitive test appear to score worse than placebo on reaction time and memory formation (possibly related to sedation) while improving the quality of memories (amount of correct memories formed, rather than 'remembering' something that didn't happen). Only one study looked at contentment, but found no influence.
Theoretically, calmness inducing agents should also help in sleep. We currently have one study suggesting that the anxiety-reducing properties can help people who have anxiety-related insomnia but beyond that most sleep research is confounded with Valerian as the two are theoretically (not yet demonstrated) additive or synergistic.
Overall, the cognitive enhancing properties of Lemon Balm appear to be somewhat overhyped. It has been shown to enhance the memory quality yet (possibly due to its sedative effects) it reduces quantititatively how many memories are formed and the rate thereof.
Currently, Lemon Balm appears to be supported for inducing calmness and relaxation but other claims may need more research.