6. Calves must not be kept permanently in darkness. To meet their behavioural and physiological needs, appropriate natural or artificial lighting shall be provided; if the latter, it must function for a period at least equivalent to the period of natural light normally available between 9.00 a.m. and 5.00 p.m. In addition, suitable lighting (fixed or portable) strong enough to allow the calves to be inspected at any time shall be available.