Following enrolment in the study, participants will be randomly assigned to a treatment start time, which will follow from an initial baseline phase. During the baseline phase, participants will wear a commercially available smart watch with physiological monitoring capability, for at least one month in order to gather behaviour frequency data, and associated physiological data; no prompts will be provided. During this period, behavioural and physiological data will be sent to a secure server where a machine learning algorithm will examine the relationships between data streams and 'self-tune' to improve its prediction of challenging behaviour episodes. In the treatment phase, participants and staff will receive a biofeedback prompt in text, or graphic form (e.g. mood light), on the smart watch/connected mobile phone, when an event has been predicted. This will prompt participants to implement a proactive anxiety management strategy, and will prompt staff to support the participant in completing this, if required. The strategy used will be predetermined in the participant's standard care plan e.g. breathing relaxation, reframing the problem, behavioural redirection. Details will be recorded using an adapted version of the Overt Aggression Scale-Modified for Neurorehabilitation (OAS-MNR), describing the situation in which the biofeedback prompt occurred, participant and staff responses to the biofeedback prompt, and any resulting behaviours, including the occurrence of challenging behaviour. Following the intervention phase, participants will return to a baseline phase for one month, in which no prompts will be provided. Again, details of any challenging behaviour will be recorded via the OAS-MNR.