344 Eur Arch Psychiatry Clin Neurosci (2011) 261:341–347 Table 2 Impact of changing the minimum episode length for hypomania for patients with bipolar disorder Hypomanic episode minimum length % Change All patients (N = 393) Mean percent days in hypomanic episode Mean percent days in depressed episode Number of hypomanic episodes Number of depressed episodes Number of patients with hypomanic episodes Number of patients with depressed episodes All hypomanic days (N = 6,188) Hypomanic days in hypomanic episode Hypomanic days not in hypomanic episode All depressed days (N = 15,699) Depressed days in depressed episode Depressed days not in depressed episode All hypomanic days bipolar I (N = 3,772) Hypomanic days in hypomanic episode Hypomanic days not in hypomanic episode All hypomanic days bipolar II (N = 2,416) Hypomanic days in hypomanic episode Hypomanic days not in hypomanic episode 4 days N 305 303 102 113 2,699 3,489 5,988 9,711 1,650 2,122 1,049 1,367 % 4.5 8.2 26 29 44 56 38 62 44 56 43 57 3 days N 491 293 145 114 3,356 2,832 5,901 9,798 2,046 1,726 1,310 1,106 % 5.9 8.0 37 29 54 46 38 62 54 46 54 46 2 days N 863 289 190 114 4,169 2,019 5,839 9,860 2,531 1,241 1,638 778 % 8.2 8.0 48 29 67 33 37 63 67 33 68 32 1 day N 2,164 264 271 108 6,188 0 5,578 10,121 3,772 0 2,416 0 % 4–2 day 4–1 day 10.6 7.3 69 28 100 0 36 64 100 0 100 0 82 -2 183 -5 86 1 54 -42 -2 2 53 -42 56 -43 136 -11 610 -13 166 -4 129 -100 -7 4 126 -100 130 -100 patients experiencing a hypomanic episode nearly doubled (102–190). When the minimum episode length was decreased to 1 day, the percent of days spent in a hypomanic episode by each patient more than doubled (4.5–10.6%), and the number of patients experiencing a hypomanic episode was more than 21/2 times greater (102–271). With a 4-day length requirement, 56% of reported days of hypomania and 62% of days of depression occurred outside of an episode. When the length requirement was decreased to 2 days, 33% of hypomanic days occurred outside of an episode. With a 1-day length requirement, there was a 13% decrease in depressed episodes for all patients, as the occurrence of brief hypomanic interrupted depressed episodes based on the algorithm. As the episode length criterion decreased, the pattern of change was very similar for patients with bipolar I and bipolar II disorder. There was no signiﬁcant difference in the percent of days of hypomania reported outside of an episode between patients with bipolar I and bipolar II disorder whether the minimum length was 4 days (P = 0.173), 3 days (P = 0.212), or 2 days (P = 0.404). When the minimum length was 1 day, all hypomania was included in an episode. With a 4-day length criterion, 102 patients experienced at least one episode of hypomania. With a 3-day length criterion, there were 43 additional patients with at least one 123 episode of hypomania. With a 2-day length criterion, there were 88 additional patients, and with a 1-day length, 169 additional patients. The comparison of the demographic characteristics of the 102 patients who experienced at least one episode of hypomania with a 4-day length, with the additional patients who experienced a hypomanic episode as the length criterion was shortened is shown in Table 3. There was no demographic that was signiﬁcantly different across all the additional patient groups. As the length cri- terion decreased, a signiﬁcantly smaller percentage of patients who had a hypomanic episode were taking anti- psychotics (4-day vs. 2-day, 53 vs. 38%, P = 0.033; and 4-day vs. 1-day, 53 vs. 39%, P = 0.026). Discussion Hypomania lasting less than 4 days occurred frequently in this study of patients diagnosed with bipolar disorder using the DSM-IV criteria. As the minimum length criterion for an episode of hypomania decreased, there was a large increase in both the number of patients experiencing an episode and in the number of episodes. When comparing a 2-day minimum length to a 4-day minimum length, there were almost twice as many patients with an episode of hypomania, and about twice as many episodes. These