If temperature is no bar in your experiment then provide temperature by blower for rapid polymerization. Low margin of variation don't affect the polymerization process. In case of SDS gel, polymerization also depends on quality and quantity of TEMED and APS. So always use new and freshly prepared 10% APS in desired concentration.
This phenomenon may be related with an accumulation of bound fluid components in your phases. See for example a reduce of the solidification(Cf. crystallization) temperature for bound water here: https://en.m.wikipedia.org/wiki/Bound_water. Alternatively, It may be a reason of forming the double electric layer