A stable mass of air holds, more or less, a fixed amount of water vapor. As the air is cooled it reaches the point where water will begin to condense (form a cloud) - this is the dew point. If this amount of moisture is fixed, the relative humidity (rH) decreases with increasing temperature, meaning the air could hold more water vapor if there were a source for additional water vapor - like from a plant leaf, say.
The relative humidity is generally maximum at night (just before the dawn, to borrow a phrase). Hence little if any moisture evaporates from the leaf.
Plant roots actively load mineral salts into their cells which creates an osmotic potential that draws in water. This can create a 'root pressure' that forces water out vein ends on the edges of a leaf. During day time, the relative humidity is generally low and any water presented as guttation evaporates before it is easily noticeable. At night and early morning, the relative humidity tends to be close to the dew point, so the guttation doesn't readily evaporate.
There is a bit more to this in terms of movement of water thru the xylem and how this reduces the 'root pressure' seen at the leaf edges when the stomata are open. Stomata tend to be closed at night (when it is dark), so there is very little or no transpiration occuring at night (independent of rH) - hence root pressure is 'fully' expressed at the leaf margins --> guttation possible.