At the height of summer the sun starts to come up around 4:45am and goes down around 9:00 pm (times not exact) and that is during DST, if there were not DST we would have sunlight from 3:45 am to 8:00 pm

here are the calculations from for Chicago, IL June 22nd, 2007

http://aa.usno.navy.mil/data/docs/RS_OneDay.html

Friday 22 June 2007 Central Daylight Time

SUN
Begin civil twilight 4:42 a.m.
Sunrise 5:16 a.m.
Sun transit 12:53 p.m.
Sunset 8:29 p.m.
End civil twilight 9:04 p.m.

If you took back the hour we sprung forward, 3:42 to 8:04pm. Basically you have theoretically moved an hour of productive sunlight to when it would be more likely used (the evening)