There are two ways that progesterone can fragment sleep, Dr Sharkey explains. During the luteal phase of the menstrual cycle – right after ovulation – progesterone levels increase, causing a higher core body temperature. “This can prompt arousals from sleep,” she says.

“Progesterone also acts on a receptor system called GABA, which is activated by things like sleep aids and alcohol. When progesterone levels are elevated, there may be an increase in fatigue and sleepiness during the day.” This is a symptom of PMT many of us will recognise (and dread). But when the progesterone levels then abruptly drop in the premenstrual or luteal phase of our cycle, we get a “withdrawal of activation” in the GABA system that can also cause sleep fragmentation, says Dr Sharkey.

Oestrogen

Although not the main offender, oestrogen fluctuations can also play a role in sleep quality and dream frequency, adds Dr Sharkey. She explains that there is research that associates oestrogen with better sleep but argues that “most of the data driving this generalisation comes from hormone replacement studies where participants were also experiencing hot flushes or other menopausal symptoms”. In other mammals, oestrogen promotes wakefulness and therefore could be a cause of sleep fragmentation near the time of ovulation, she suggests.

Keep having vivid nightmares? Your hormones could be to blame – here’s why