Diversity of daily METs. This chart displays time patterns of MET values over a 24-hour period for a random subsample of 2000 person-days reported by male GSS respondents. The vertical dimension represents time of day plotted minute by minute from 4:00 am to 3:59 am (bottom to top), while the horizontal dimension represents individual, sample person-days. The person-days were clustered so that adjacent person-days on the chart were as alike as possible (the tree at the top of the graph displays the linkages among respondent clusters). The shades of grey in the chart code for METs on a log scale with white corresponding to low values (e.g., sleep at the beginning and end of each day) and black corresponding to high values (e.g., vigorous activity at work or in sports). There do not appear to be any compact, widely separated groups, and clustering appears to be determined as much by timing (e.g., shift workers, or those with vigorous activity in the morning and daytime) as it is by intensity.