8 42. Custody by the police is in principle of relatively short duration. Consequently, physical conditions of detention cannot be expected to be as good in police establishments as in other places of detention where persons may be held for lengthy periods. However, certain elementary material requirements should be met. All police cells should be of a reasonable size for the number of persons they are used to accommodate, and have adequate lighting (i.e. sufficient to read by, sleeping periods excluded) and ventilation; preferably, cells should enjoy natural light. Further, cells should be equipped with a means of rest (eg. a fixed chair or bench), and persons obliged to stay overnight in custody should be provided with a clean mattress and blankets. Persons in custody should be allowed to comply with the needs of nature when necessary in clean and decent conditions, and be offered adequate washing facilities. They should be given food at appropriate times, including at least one full meal (i.e. something more substantial than a sandwich) every day.1 43. The issue of what is a reasonable size for a police cell (or any other type of detainee/prisoner accommodation) is a difficult question. Many factors have to be taken into account when making such an assessment. However, CPT delegations felt the need for a rough guideline in this area. The following criterion (seen as a desirable level rather than a minimum standard) is currently being used when assessing police cells intended for single occupancy for stays in excess of a few hours: in the order of 7 square metres, 2 metres or more between walls, 2.5 metres between floor and ceiling. 1 The CPT also advocates that persons kept in police custody for 24 hours or more should, as far as possible, be offered outdoor exercise every day.