Here's why:

* Energy Source: Solar-powered clocks rely on solar panels to convert sunlight into electricity. This electricity powers the clock's internal mechanisms.

* Lack of Sunlight: When there is no sunlight, the solar panels cannot generate electricity.

* Battery Backup (Limited): Some solar-powered clocks have a small battery backup. This battery will allow the clock to run for a limited time in the absence of sunlight. However, the battery eventually runs out, and the clock will stop working.

However, there are some exceptions:

* Hybrid Clocks: Some clocks are hybrid, combining solar power with a traditional battery. These clocks can run even when there is no sunlight, as long as the battery is charged.

* Radio-Controlled Clocks: These clocks receive time signals from radio towers. They don't rely solely on solar power and will work even in darkness.

In essence, a traditional solar-powered clock requires sunlight to operate. If you need a clock that works in the absence of sunlight, look for a hybrid or radio-controlled model.