Over the counter sleeping pills are H1-antagonists!
We have demonstrated that the activation of H1-receptors can shift the thalamus from a sleep mode to a waking state. It makes sense, therefore, that the block of these receptors, such as by over-the-counter sleeping pills, will result in the promotion of slow wave sleep!
see McCormick, D.A. and Williamson, A. Modulation of neuronal firing mode in the LGNd by histamine: Possible cellular mechanisms of histaminergic control of arousal. Journal of Neuroscience (1991) 11:3188-3199.