This article is part of "Safe Sesh", a VICE harm reduction campaign produced in collaboration with The Loop and the Royal Society for Public Health. Read more from the editorial series here.

There's no sadness like a comedown sadness. That feeling of waking up after a big weekend and knowing that today is going to be the worst day of your adult life; that your brain will be mush, capable only of making you tear up at memes and reconsider every one of your life choices thus far. It is the terror that would be too awful to be named, if some serotonin-sapped pioneer hadn't come up with the term all those years ago.

Regardless of all this – the hours and days we spend lost in a funk of brain zaps, endless sniffing and existential angst induced by something as benign as Summer Heights High – we still love taking comedown-inducing drugs. Nearly 500,000 people in the UK took ecstasy in 2015/2016, and 3.3. percent of people aged 16 to 59 are considered frequent drug users.

To try to understand the science behind the sadness I called Guy Jones with a list of common comedown symptoms. Guy is the Technical Lead at Reagent Tests UK and volunteers with harm reduction groups like The Loop. He's also a chemist and knows drugs inside out.

1: That awful scratchy, runny nose that your boss has noticed and you've palmed off as a cold, again.

"Cocaine is a vasoconstrictor, and the blood vessels in the nose have been temporarily shrunk by the cocaine. As a rebound mechanism the body has said, 'The blood vessels in the nose are too small, so let's send a hormone or a nervous signal to dilate those blood vessels.' By the time the cocaine wears off, the body is overcompensating and the nose becomes congested."

2: Those terrifying brain zaps that feel like someone's sporadically tasering the inside of your head.

"These are quite hard to fully understand because there's a lack of research done around MDMA, but it's a fairly common phenomenon that people describe, especially after festivals. Interestingly, they are also reported by people withdrawing from SSRI antidepressants, which increase serotonin levels by preventing recycling of serotonin, while MDMA works by simply producing more. Therefore, it's very possible they are linked to the body trying to re-address a serotonin imbalance. Supplementation of 5HTP can help this for many people, although you should not take it until after using MDMA."

[Read some more about brain zaps and how to counter them here.]

3: Awful nightmares about dead relatives or witches or just bad, terrible things happening to your genitals; dreams so vivid you sleep with the light on, even though you're a relatively rational adult.

"Your body will be trying to catch up on its REM sleep, as chemicals hugely disrupt this and stop you from getting the required amount [around 20 percent of your sleep would normally be REM]. It's during REM sleep that we are most active, so it's more likely you'll be flipping in and out of sleep and remembering your dreams. Serotonin plays a central role in sleep and dreaming, so it's not surprising that if you have an MDMA-related serotonin depletion – especially if you've been taking it for multiple days at a festival – there's some interference in your sleep patterns."

4: You've got the shakes and you're 21 years old.

"It could be related to neurotransmitter issues with the brain; it could be related to the heavy effects of alcohol use and, or sleep deprivation. Either way, it's a perfect storm. If your body was ever going to develop tremors it would probably be now."

5: It's Sunday. An episode of Come Dine With Me just made you cry onto your Papa John's. This is the absolute fucking limit.

"With MDMA, the brain is crazily over-saturated with [mood regulator] serotonin and other neurotransmitters, and it thinks, 'This isn't right, I'm going to turn the volume down for a bit,' and it takes a while to turn that volume dial back up again – so you're very low on serotonin. Also, you'll likely have sleep-depletion, which would have an affect on your mood regardless of you taking any drugs."

6: You get home from work on Monday and order your third Deliveroo in two days. Will you ever want to stop eating crap food?

"Stimulants reduce appetite, so it seems entirely reasonable that in the days following a big night your body would say, 'We're running a bit low here, can we top up the calories?' Try to find some balance, though – eat some fruit, vegetables and oily fish."

7: It's Tuesday and you can't snap out of the sadness; you just spent a lunchtime reading old emails from your ex and don't think you'll ever experience joy again. Do you need to call a doctor?

"MDMA binds to and inactivates an enzyme in the brain called tryptophan hydroxylate. That enzyme is responsible for converting tryptophan in the diet, which is then converted into serotonin, so without that enzyme the body is temporarily unable to produce as much – or any – serotonin as it normally would.

"The body stopped producing serotonin on the day you took [the MDMA], but it has some reserve left, so that next day there is a little bit of serotonin there. By the time you get to the second or third day, in theory, no serotonin is left and the low reserves that were left are increasingly used up, so you reach a kind of minimum level while the body is still rebuilding the enzyme that has been inactivated by the MDMA. There's no formal studies on how long it takes to rebuild the enzyme, but a sensible estimation is that it's about a week to restore it to full levels.

"Combine this with a possible sleep-deficit and general anxiety about going back to work and dealing with 'the real world', and you've certainly got a recipe for feeling very low."

8: The insides of your mouth feel like they've been scraped with a Brillo pad. A kiwi just made you wince. State of you.

"The technical term for jaw clenching is 'bruxism', and stimulants instigate this natural tension in the jaw muscles that some people will perhaps chew their mouth to relieve. What doesn't help is that stimulants reduce the amount of saliva reduction, so you get a dry mouth and don't have so much of the natural protection and lubrication that you normally would."

9: Your friend who was getting mashed with you has been totally fine. She's just called you from the gym and is now talking about going ice skating tomorrow. Is she just one of those awful, mentally-strong people, or is there something wrong with you?

"Everybody has different neurology. Some people are more anxious, some have different enzymes in the brain or liver. Some people feel the effects more strongly or less strongly, so I think it would be a reasonable conclusion to think that her neurology is such that she doesn't feel the comedown so severely. Some people are just lucky, depending on the way you look at it."

Yep, I guess so. Thanks, Guy!