I agree that change in rhytm of ingestion wheatgrass juice could cause change in the mood. If you took juice at the evenening and it was not with full stomach
( last meal around 6pm ) and wheatgrass shot at around 9pm could cause more deeply cleansing effect. The core of the cleansing and tiredness is in the liver activity.
Liver is more concetrated on cleansing the toxins from the past and that makes you tired but feeling pleasant tired and sleepy ( there is great need for oxygene ).

At 5am in the morning it is started so called liver cycle. Where liver is most intensive organ in the body. In that period most intensiven cleansing processes in liver are going on.