Blakemore (1988) studied workers at a chemical plant in Utah who worked a three weekly schedule: the first week a night shift, the next week an evening shift and the final week a day shift. This is the most common shift pattern and lab animals who are subjected to this kind of rotating pattern of light and dark suffer from increased heart disease and a shorter life span. This shows how the circadian rhythm is consistently disrupted by having to switch between day and night shifts which disrupts the link between exogenous zeitgebers and biological rhythms. This particular shift pattern is more difficult to adjust to due to as the workers are experiencing phase advance as their circadian rhythm is reduced and they need to go to sleep and wake up earlier.