Diurnal preference towards eveningness among adults has been associated with unhealthy habits and a range of health hazards, such as sleeping problems and higher odds for depression. We wanted to analyse whether diurnal preference towards eveningness is associated with more severe symptoms regarding sleep problems and mental disorders among young adults. Our sample consists of 469 young adults, aged 18–29 years, from the Mental Health in Early Adulthood Study in Finland (MEAF) conducted in 2003–2005. Chronotype was based on the assessment of one question that was asked first in 2000–2001 and the second time in 2003–2005. Those 73 participants who changed their chronotype were excluded from the main analysis, but separate analyses were performed with this group. Concerning sleep, E-types reported higher dependency on alarm clocks (p < 0.001), and E-types and I-types had more problems in feeling refreshed after waking up (p < 0.0001 and p < 0.05 respectively) than M-types. Regarding mental health, E-types and I-types had lower odds for any lifetime DSM-IV Axis I disorder (p < 0.05 and p < 0.01 respectively) than M-types. Our results are in line with previous findings that those with the diurnal preference towards eveningness have more frequently three or more lifetime mental disorders, more sleeping problems, more seasonal variation in mood and behaviour, and more burnout compared with those with the diurnal preference towards morningness.