If the starting dose turns out to be insufficient in 5 patients per group it will be increased in the following patients of that group. The effect of an initial propofol dose is tested to find the optimal propofol doses for neonates in different developmental stages. The study will be continued until the appropriate dose for each age group is determined. The appropriate doses are re-used in another 5 patients per age group to validate the predetermined doses. Safety is monitored very intensively and if hypotension occurs this is immediately treated. The patient will benefit from this intensive safety monitoring, because side-effects will be detected earlier and can be more effectively treated.