Travel to work area legal status. However, they give planners and geographers an alternate view of urban life as their boundaries are tied not to arbitrary administrative limits but socio-economic ties. Having an idea of where people commute from for work is particularly useful for public transport planning. Travel to Work Areas are defined by the Office for National Statistics using census data for commuting between wards, based on the different locations of individuals' home and work addresses. A Travel to Work Area is a collection of wards for which "of the resident economically active population, at least 75% actually work in the area,
Shift work may be due to alterations in circadian rhythm: melatonin, a known tumor suppressor, is generally produced at night and late shifts may disrupt its production. The WHO's International Agency for Research on Cancer listed "shiftwork that involves circadian disruption" as a probable carcinogen. Shift work may also increase the risk of other types of cancer. Shift work also increases the risk of developing cluster headaches, heart attacks, fatigue, stress, sexual dysfunction, depression, dementia, obesity, metabolic disorders, gastrointestinal disorders, musculoskeletal disorders, and reproductive disorders. Shift work also can worsen chronic diseases, including sleep disorders, digestive diseases, heart disease, hypertension, epilepsy, mental