I am slightly confused in that you said it only happens at night and is gone in the morning.
Do you mean that the swelling occurs throughout the day and during the night it reduces and is gone in the morning?
Excess water and damage of the 'check valves' in the veins of the lower legs and/or some blood pressure issues can inhibit the return flow of blood UP from the legs. This can cause an amount of 'seepage' from the veins and fluid accumulates in the flesh as the
? searching for the word,, the lymphatic system is over-whelmed and unable to absorb and remove all the excess 'seepage'.
During the night while laying flat the return pressure is more 'normalized' and the lymphatic system manages to catch up.
For many this may be a sign that some sort of support hose may be required to 'squeeze' the lower leg. This helps to reduce that swelling.
You need to see a vascular specialist and discuss this. As far as I know, most of the tighter support hose are prescription only.