Whether it's jumping out of a cupboard dressed as the grim reaper or simply saying 'boo', millions will be giving their loved-ones a fright this Halloween. 

But if you're planning a prank, researchers warn there's a 'sweet spot' to hit – where you provide a humorous fright without overstepping the mark. 

In a new study, researchers at Aarhus University in Denmark claim that the best scary Halloween capers fool people only for a second or two. 

They found that humour and fear are closely intertwined in the human brain – so it's important the victim can see the funny side sooner rather than later. 

Lead author Marc Hye-Knudsen told MailOnline: 'To me, the best scare is a scare that your friend can also find funny afterwards. 

'That requires knowing your audience – your scare has to be big enough to elicit a reaction but not so big that it’s no longer harmless. 

'Don’t dress up like a serial killer and hide in your grandmother’s closet if you know it’s liable to give her a heart attack. 

'But also, don’t just walk up to your friend and say “boo” either as that’s not going to be very interesting for either party. 

Dr Hye-Knudsen added: 'Anticipate what level of scary your target is going to be able to laugh at afterwards, and when you laugh at their scare, invite them along to laugh at it so that they don’t feel they’re being laughed at but instead being laughed with. 

'In that way, it’s not just funny for you, it’s also funny for them.

'In other words, help them to find that “sweet spot” of a scare they’ll actually enjoy.'

It's already well known that the evolutionarily purpose of fear is to set us into a state of action-readiness to deal with a potential threat. 

For this reason, it involves a cascade of stress hormones that activate our sympathetic nervous system, preparing us to fight or flee. 

But the researchers say that fear is 'deeply connected' with humour, partly due to how close they are processed in the brain. 

For the study, published in Evolutionary Psychology, the researchers reviewed ways in which scary things are rendered humorous – including 'jump scares', peekaboo and comic horror films. 

A classic prank, the 'jump scare' is when someone suddenly jumps out from a hiding place and loudly says 'boo!', 'argh!' or similar. 

The jump scare often elicits a response such as a scream or shout, shortly followed by shared laughter between the two. 

'The victim needs to go through a process of cognitive reappraisal to find humor in the prank,' said Dr Hye-Knudsen. 

Meanwhile, peekaboo, typically played with infants, can be startling and scary to the youngster, but they gradually learn to find it humorous. 

The victim of a sudden scary prank will initially consider it a 'violation' of the world they consider safe, before quickly realising it is not real or 'benign'. 

'Scary situations constitute a violation, so anything that scares us can be rendered humorous if it is ultimately appraised as benign,' said Dr Hye-Knudsen.

Essentially, a good approach may be not to take the prank too far by making the violation of their safe world last very long.

Doing so could have nasty long-lasting effects, such as triggering a heart condition, creating psychological damage or more. 

In October 2010, a high school teacher in Taunton, Massachusetts decided it would be funny to enter his classroom wearing a mask and wielding a chainsaw. 

Unsurprisingly, his pupils were terrified and in a mad rush for the classroom exit one child suffered a fractured bone and serious knee injuries, triggering a lawsuit from the parents. 

In another instance, parents in Illinois played an ill-considered prank on their children pretending the mother had been killed by 'Michael Meyers', the antagonist of the slasher film series Halloween.

Unfortunately there was a Meyers family in the area and the distressed children called the police.

Of course, any prank should be within the accepted boundaries of what's acceptable, not to mention lawful.  

Once you've settled on your prank method, the study authors stress to make sure your victim is someone who would see the funny side. 

'Contextual cues, cognitive reframing, and psychological distance can be used to render things that would otherwise scare us benignly humorous,' said Dr Hye-Knudsen. 

'Becoming aware of this can help us harness the power of humor as a tool for regulating fear. 

'This can be part of the process of finding the sweet spot of fear during recreational fear activities – but it can also be part of a therapeutic practice.'