In the FAA JO 7110.65 that provides rules for air traffic control, the following limitations are listed for VFR climb and descent:
8−8−5. VFR CLIMB AND DESCENT
a. In the Houston, Miami, and San Juan CTAs, IFR flights may be cleared to climb and descend in VFR conditions only:
1. When requested by the pilot; and
2. Between sunrise and sunset.
The key part here seems to be the second condition, as section 7-1-2 just lists the first one.
7−1−2. VFR CONDITIONS
a. You may clear aircraft to maintain “VFR conditions” if one of the following conditions exists:
1. The pilot of an aircraft on an IFR flight plan requests a VFR climb/descent.
So what makes the Houston, Miami, and San Juan areas special that VFR climb/descent must be during the day? Section 8 is the Caribbean ICAO region, which seems to only include the oceanic sections of Houston and Miami. But this rule doesn't specify "oceanic" and none of the other oceanic areas seem to have this rule.