Aim The aim of this critical care sleep assessment pilot study was to evaluate the usefulness of three sleep assessment tools to identify which, if any, provided the closest comparison between the nurses’ judgement and the patients’ experience of their sleep. The study objectives were to: (i) compare patients' and nurses' assessment of sleep using three different rating tools. (ii) Ascertain patients’ preferences with non-interventional, user friendly, practical tools in critical care. (iii) Recommend changes and improvements to the way that sleep is assessed and documented. Background Sleep is important for promoting critical care recovery and sleep disturbance is known to cause irritability, aggression and increased stress levels. The availability and use of valid critical care sleep assessment tools is limited. Design A descriptive comparative study using three sleep assessment-rating scales were constructed to provide easy to understand tools for completion by both patients and nurses in critical care. Methods Structured interviews were undertaken with 82 patients and 82 nurses using a convenience sample from four multispecialty critical care units in one large teaching trust. Patients were included in the study if they met a list of pre-defined criteria to obtain responses from lucid orientated patients. Results No tool produced a close association between the nurses’ assessment of the patients sleep and the patients’ assessment of their sleep. Patients found two of the three tools easy to use when rating their sleep. Discussion Objective invasive measurements of sleep as well as complex subjective tools appear inappropriate to be used as a part of daily critical care practice. The application of simple rating scores has a high degree of error when nurses assess patients’ sleep, even though high levels of patient observation and assessment are practiced in critical care. Conclusions More research is needed to examine the assessment of sleep in critical care, particularly linking rating scales to alternative methods of physiological assessment of sleep. Findings indicate nurses are unable to accurately assess critical care patients’ sleep using rating assessment tools. However patients were found to prefer two sleep assessment tools, one banded in hours to assess sleep quantity and one as a comparison against normal sleep to assess sleep quality. Relevance to clinical practice This study reviews the importance of sleep assessment and the diverse methods available for assessing sleep focussing on the critically ill patient. More noteworthy it highlights how nurses sole judgements of patients sleep is not a reliable method in clinical practice, however it provides some indication on the application of ‘easy to use’ tools to assist in the patients assessments of their sleep.