This thesis is submitted for the Clinical Psychology Doctorate at the University of Birmingham. The thesis contains three chapters which comprise the research component of the doctorate. The first chapter is a systematic review of the concordance between objective and subjective sleep assessment methods in rare neurogenetic syndromes. The second chapter is an empirical paper which uses an existing actigraphy dataset to explore the impact of sleep assessment methodologies on sleep parameter estimates across neurogenetic syndromes. The final chapter is in the form of a press release for both the review and empirical chapters.