In our increasingly hectic lives, the chance to have a lie-in is a dream come true for many of us.
But sleeping for too long can actually cause nightmares, research suggests.
More than nine hours of shut-eye doubles your risk of having a bad dream compared to getting six hours, an Oxford University study found.
The scientists said this is because nightmares occur during the rapid eye movement (REM) stage - a type of sleep that occurs in intervals during the sleep cycle - and so if you spend longer asleep, you will go through more stages of REM.
The researchers suggested cutting down on sleep if you experience a lot of nightmares, with seven to nine hours the optimum amount.
Dr Bryony Sheaves, a research clinical psychologist at Oxford’s Sleep and Circadian Neuroscience Institute, said: "The longer that we sleep, the more rapid eye movement (dreaming sleep) we get. This means that the opportunity for nightmares to occur is greater.
“Some people may have noticed they have more dreams on days that they sleep in. If you are particularly prone to nightmares, this may be placing you at a greater likelihood of having one."
One in 20 people have a nightmare every week. During the night, we spend much of the time in a deep, restorative sleep - also called non-REM sleep - when breathing is slower and muscles relaxed.
This is interspersed with intervals of REM, which occur around every 90 minutes, during which your eyes move rapidly under your eyelids and you have spikes in brain activity as you dream. REM intervals become longer later in the night, dominating the second half of your sleep period.
The team, who questioned 846 people aged 18 to 77, found 45% had suffered at least one nightmare in the previous fortnight.
They examined what affects the frequency of nightmares, looking at sleep duration, alcohol consumption, exercise levels, worry, paranoia, hallucinations and depersonalisation, a kind of psychological shutdown caused by experiencing trauma.
Sleep duration was the second-biggest risk factor after worrying. It is believed the repetitive thoughts triggered by anxiety “feed” negative dream content. The distress caused by nightmares can then create a vicious cycle.
Paranoia, hallucinations and depersonalisation were also found to be potential triggers for bad dreams. But alcohol and a lack of exercise were found to have no effect.
The results were adjusted to reflect the differences in people’s stress levels and experiences of trauma.
The study, published in the journal Social Psychiatry and Psychiatric Epidemiology, states: “We predict that stabilisation of the sleep window (ie reducing a long sleep duration) might thus lead to a reduction in nightmare occurrence for those with longer sleep durations.”