Unfortunately when she may feel she needs her sleep the most, it too is changing. Both the physical and emotional changes which accompany pregnancy interfere with sleep. There is an increase in the hormone progesterone. Progesterone causes sleepiness. Frequent awakenings to urinate which cause interrupted sleep.
Interrupted sleep means daytime sleepiness.
The good news is that this allows for a better quality sleep than during the 1st trimester. The fetus has moved and is now putting less pressure on the bladder so there are less nightly bathroom breaks. However, sleep quality is still not what is was before pregnancy. Most pregnancy related problems happen now. Physical discomfort disturbs sleep most.