Rocking movements are known to affect human sleep. Previous studies have demonstrated that the transition from wake to sleep can be facilitated by rocking movements, which might be related to relaxation. However, it is not yet known which movements would have the greatest effect. Thus, a 6-degree-of-freedom tendon-based robotic bed was developed, for systematic evaluation of vestibular stimuli. The applicability of the device was evaluated with 25 subjects. Six movement axes were tested and analyzed for differences in promoting relaxation. Relaxation was assessed by electroencephalogram, electrocardiogram, respiration and a questionnaire. The developed device fulfilled all needed requirements proving the applicability of this technology. Movements had no significant effects on the electroencephalogram and electrocardiogram. Respiration frequency was significantly lower for baseline measurements without movement (median 0.183-0.233 Hz) compared to movement conditions (median 0.283-0.300 Hz). Questionnaire ratings showed a trend (p = 0.057) toward higher relaxation for movements along the vertical axis (z-axis) (median 4.67; confidence interval 4.33-5.67) compared to the roll-axis (median 4.33; confidence interval 3.67-5.00). Movements along the vertical axis (z-axis), therefore, appear most promising in promoting relaxation, though no effects were found in electroencephalogram and electrocardiogram variables. This lack of effect might be attributed to the short exposure to the movements and the large inter-individual variability and individual preferences among subjects.