Separating treads

The staircase in my house is hardwood. The treads on the staircase are coming apart, and some have gaps up to 1/8" wide.

These gaps are not noticeable during the day. But at night, any light in the basement shines right through. A convenient reminder that light needs turned out when we head upstairs for bed.

One could consider it an energy-saving feature. It may not completely compensate for having one room of the house with holes to the outside, but every little bit counts.