According to new research, it may be the latter, because scientists have found that sleep actually consolidates bad memories, making you live with them for longer.
Sleep's role in helping us consolidate our memories is vital in how we process and store useful information, but a new study shows that the same principle applies to negative thoughts – if we fall asleep with them, they can be harder to forget in the long term.
"This study suggests that there is certain merit in this age-old advice: 'Do not go to bed angry'," researcher Yunzhe Liu, who conducted the study at Beijing Normal University in China, told AFP.
"We would suggest to first resolve (an) argument before bed."
Liu and fellow researchers recruited 73 male college students to test how well they could intentionally suppress negative memories, and to figure out how sleeping might impact the process.
The participants were trained to associate images of neutral faces with disturbing images of things like injured people, mutilated bodies, and crying children.
The next day, after a full night's sleep, they were shown the neutral faces again and instructed to either recall the negative associations, or to try to suppress them, using a psychological technique called think/no-think.
In this technique, when participants are asked to 'think' of something, they actively try to recall it – such as the associations learned between the faces and the negative imagery.
By contrast, when they are asked to 'not think' of something, they consciously try to avoid thinking of it – with research showing this task can actually help people to forget things they've learned.
Then, in a separate session, the group learned associations between a new set of neutral faces and negative images.
This time, their think/no-think testing session was conducted just 30 minutes later – as opposed to 24 hours later in the previous session – to see if reconciling the bad memories early could help the participants forget.
The results showed that the participants' suppression efforts were more effective after 30 minutes than when attempted 24 hours later, suggesting that a night's sleep had helped to consolidate the negative memories, making them harder to intentionally inhibit later.
Brain scans with fMRI machines during the recall/suppression exercises showed why this might be happening.
When the participants tried to remember or inhibit the memories only 30 minutes after learning the associations, their neural activity was centred in the hippocampus – the part of the brain primarily associated with memory.
But after a night's sleep, the memory task showed that neural activity was reduced in the hippocampus, having dispersed among a number of cortical regions also associated with memory and information processing, including the lateral parietal cortex, angular gyrus, and middle temporal gyrus.
The researchers think that once the hippocampus has encoded these memories in short-term memory, they get separated like this and distributed to other parts of the brain to facilitate more long-term storage – which in the case of negative thoughts, might not be a good thing.
"Overnight consolidation makes aversive memory more resistant to suppression by promoting hippocampal-neocortical reorganisation of the memory," Liu told Katherine Lindemann at ResearchGate.
In other words, if you want to forget about something unpleasant, it may be better to try to resolve how you feel about it or move past it before you go to bed, as those memories could spread deeper and wider in your brain if you don't.
It's worth pointing out that the study has some limitations, because only men took part in the research, and the sample size was very small.
It's also possible that the reduced ability to suppress memories may be a result of extra time having passed (24 hours as opposed to 30 minutes), and not solely due to the act of sleep.
But the findings could tell us a lot about how negative experiences become entrenched in the mind, which could help researchers develop better treatments for conditions like post-traumatic stress disorder.
"The results are of major interest for treating the frequent clinical problem of unwanted memories, memories of traumatic events being the most prominent example," neuroscientist Christoph Nissen from the University of Freiburg in Germany, who was not involved with the study, told New Scientist.
Of course, using memory suppression techniques directly after people experience emotionally traumatic events won't be possible in many cases, but the research could still help scientists to develop new kinds of reconsolidation techniques – in which old memories are modified through psychological interventions – and might spur research into other treatments too.
"For example, sleep deprivation immediately after traumatic experiences may prevent traumatic memories from being consolidated into stabilised representations and thus provide the opportunity to block the formation of traumatic memories," the researchers write in their paper.
As for the rest of us, who may simply not want to wake up feeling grumpy, Liu's advice is simple.
"We suggest that people try to get a bad memory out of their minds as soon as they can," he told New Scientist, "not to think about it too much, and especially not to sleep on it."
The findings are published in Nature Communications.
Thank you Science Alert for this awesome article.