Pregnancy often brings on a multitude of sleep disturbances including nausea, heartburn, restless leg syndrome, and snoring. These sometimes unavoidable problems may be aggravated by bad sleeping habits that you established before you were pregnant.
During the initial three months of your pregnancy you are likely to feel surprisingly drained and fatigued. During this time your sleep-inducing hormone progesterone starts surging through your body. You may notice that you need more rest during the day and more sleep during the night. This is due to the new demands pregnancy is making on your system. Your body is trying to cope with the dramatic changes in the hormone levels. During the second trimester, your body will get more used to the changes.
In the third trimester, again, particularly from about six weeks before your baby is born, you will feel quite exhausted and will feel the need for more rest, although your physical strength might have increased progressively, remember that your baby is now becoming heavier, it is growing and developing all the time, and its movements are more noticeable now. That is the reason for feeling fatigued as also the need for additional hours of rest and sleep.

It is quite common to have sleep difficulties during pregnancy. There are several reasons for this.
Increase in metabolism — Your baby is growing and developing inside your womb all the time—around the clock. So its metabolism doesn’t slow down when evening comes—or when it is time to sleep for you. Your body needs to supply food and oxygen all the time for its regular growth. Your wakefulness could be a result of that.
Indigestion – Whether you are pregnant or nor, indigestion always creates sleep problems. So, during pregnancy you need to be more careful about it as indigestion and heartburn are very common during this period.
Frequent urination – Your growing uterus puts pressure on your bladder because of which you feel like peeing more frequently. This also makes you wake up in the middle of the night.
Cramps – Many women complain about leg cramps that are bad enough to disrupt sleep.