MODEL RELEASED. Sleep research. Sleeping man being monitored during research into sleep disorders. Electrodes measure his brain activity, while his heartbeat and blood oxygen levels are also checked. Several factors can affect sleep, including drugs (such as caffeine), stress and environmental factors. Medical conditions include sleep apnoea, the temporary cessation of breathing during sleep. The lack of oxygen this causes makes the patient wake suddenly, severely disturbing sleep. The causes of the condition are unknown. Although an essential function, sleep is poorly understood.