Article by my colleague Tadas Stumbrys:
https://archiv.ub.uni-heidelberg.de/ojs/index.php/IJoDR/article/view/9483/pdf_30
Abstract: Lucid dreams – dreams in which the dreamer is aware that is dreaming – most frequently occur during REM sleep, yet there is some evidence suggesting that lucid dreaming can occur during NREM sleep as well. By conducting a sleep laboratory study on lucid dreams, we found two possible instances of lucidity during NREM sleep which are reported here. While lucid dreaming during NREM sleep seems to be much rarer and more difficult to achieve, it appears to be possible and is most likely to occur during N1 sleep, somewhat less likely during N2 sleep and yet to be observed during N3 sleep. Future studies should explore induction methods, underlying neural mechanisms and perceptual/dream content differences between REM and NREM lucid dreams. Furthermore, a consensus agreement is needed to define what is meant by lucid dreaming and create a vocabulary that is helpful in clarifying variable psychophysiological states that can support self-reflective awareness.