I guess it depends on the route the half hour bike commute takes you thru, but 30 minutes twice a day is generally doable. It could help you fall asleep a night if the ride helps get rid of excess energy from work day stress. My old commute was about 45 to an hour by bike and it helped me be a nicer person to my family. My previous crummy job was not only more bearable between bike rides but also gave me the emotional strength to quit it a year after I started bike commuting. Gawd that crappy job was pure, unadultered misery on it's on but bracketed by 40 minutes trapped in the TTC, it was a daily horror.