There are no norms that they should eat cooked rice at night. During days like Ammavasai, it is specified for a few to take tiffin. However, Sumangali women are exempted from this.
Some households still follow the practice of women, children eating rice at night. As they feel on those days, where Sumangali Women and children should not eat palakaram(tiffin) alone, and partaking tiffin may get counted as palakaram. To avoid that they are having the habit of eating rice at night. Though this is not stipulated by scriptures, it is not wrong to follow this practice as a cautionary approach.