According to the crime reports that I read (and I do keep up on them, but for my county), most incidents happen during daylight hours.
There apparently is a flasher/potential rapist that is "working" the W&OD in Fairfax County (Vienna to Reston/Herndon). If I'm not mistaken, he has attacked either during daylight hours, too (not 100% sure of this). There were about 3 similar attacks.
The trail is USUALLY safe. A lot of commuters ride it, people on nightshift. Or people who have no other choice about when to ride/run/use the trail.
Pam, there are plenty of wooded/isolated spots along the trail. I think it's mostly unlit out that way, too. I've seen cops on or in the vicinity of the trail during the dark hours.
And to be honest, I've seen plenty of hoodlums on the trail during the day. One must stay alert, smart, don't wear an iPod, carry a cellphone. I always have pepper spray with me, regardless of when I choose to ride.