Despite its name, morning sickness can occur at any time of the day or night. The Hormonal Culprit: GDF15Recent studies have pinpointed the hormone GDF15 as a primary driver of morning sickness. The amount of GDF15 circulating in a pregnant woman’s bloodstream directly correlates with the intensity of morning sickness. Hyperemesis gravidarum is an extreme form of morning sickness, characterized by relentless vomiting and nausea throughout pregnancy. But it’s often overlooked by doctors – probably because morning sickness is so common.