An intravenous (IV) cannula will be inserted into the elbow area. At least one hour after the IV cannula is placed, but usually at 16:30, a blood sample (2ml) will be taken through the IV cannula and the cannula flushed. At 22:30 the main lights will be switched off and the volunteer encouraged to sleep. In total, 20 samples will be taken from the cannula over 24 hours.