Plants require a dark period in which to focus on cellular respiration rather than photsynthesis. A dark period is doubly inportant for orchids however, as most orchids are
CAM plants, which means they only absorb CO2 at night (in darkness), storing it as malic acid which is then used during the day during photosynthesis (unlike most plants which absorb CO2 during the day and directly use it during photosynthesis). CAM Photosynthesis is an adaptation meant to conserve water, as the plant's stomata are only open during the night resulting in less water loss to evaporation. Epiphytic plants, succulents, and cacti are the most typical examples. You can read more about it
here, or just google 'CAM photosynthesis'.
The upshot is that an orchid deprived of a dark period would stop producing the materials necessary to grow and keep life functions going.