Hi Rachel,

The rationale behind it is that fish oil can help to eliminate "the munchies".  It helps to prevent hunger by inhibiting hormones called endocannabinoids (eicosanoids that cause increased hunger) from binding to their receptors in the brain.  As I understand, it will only have this effect for a short number of hours after it's ingested (has to do with how long EPA exists in the brain before being converted to DHA).  Since evening is the prime time for some individuals to become carried away with snacking, having fish oil with the evening meal can give you the extra boost you might need to prevent evening snacking.  You can read more about it in Toxic Fat.

See what works best for you.  I'm one of those who does better taking fish oil earlier in the day.  I find my sleep is restless if I take it at dinner time or later.  My son (adult age) on the other hand, can take his high dose at bedtime and sleep like a baby.