Chris Seton, a paediatric and adolescent sleep physician at the Woolcock Institute and Westmead Children's Hospital, also warned the US experience of using prescription drugs before exams such as Ritalin and Modafinal was trickling down to Australian classrooms.

"The futility of it is they are trying to wake up their tired brain to study late. It's a bit like taking Panadol for a headache and hitting your head against the wall," Dr Seton said.

On Monday, the HSC will enter its third week as up to 10,000 students prepare to sit the  ancient history exam.

Dr Seton said this time of year was a peak for students suffering sleeping problems, many of whom take prescription drugs.

"The kids I see in year 12 at this time are in crisis," said  Dr Seton.

"You see it through their sleep deprivation, it is the spring time peak. I hate the word epidemic, but I don't know what a better word is."

Ritalin is used to treat attention deficit disorders, and Modafinil is used for sleep disorders such as narcolepsy. Both have been linked to illicit use as "smart drugs" or cognitive study aids in the US and Australia.

Dr Seton said he was now seeing melatonin, a hormone naturally released at the onset of darkness, being taken in tablet form as sleep deprived students tried to reorientate their body clocks.

The drug, available through prescription for up $100 a bottle in Australia, is increasingly available through online marketplaces selling for as little as 50 cents a tablet.

"The use of melatonin, energy drinks and stimulants goes in parallel, they are the things people take for sleep deprivation," he said.

He said many parents seek help for their children when it was already too late  and urged them to seek sleep treatment before the HSC season starts.

Medicare data has shown more than 24,000 people under the age of 19 are on ADHD medications in NSW, while prescriptions for Ritalin rose 300 per cent over the past decade.

A US study in the journal Brain and Behavior in 2012 found beyond cannabis, prescription stimulants accounted for most of the illicit drug use on college campuses by 2004.

Dr Seton said HSC students with ambitious parents paying expensive school fees were over-represented among students he saw using prescription drugs.

"There is a skew towards private independent schools," he said.

Students have long openly discussed purchasing drugs such as Ritalin and Modafinil on internet forums for as little as $5 in the lead-up to the HSC.

"I know some ppl [sic] take caffeine or smoke a cigarette before exams but what about harder shit, what about methamphetamines, dexedrine, adderall, ritalin etc?" asked a student on the Bored of Studies forum.

One student said they got "Ritalin for my half yearly's and it was like me and that test paper were the only thing in the universe".

More sinister drugs are on the radar of students too. One student suggested using No-Doz caffeine pills to help stay awake after class. "Or just use meth," another said.

Last year it was revealed drug-related offences on NSW school grounds were at their highest in more than a decade, with an average of one incident a day in 2014, mostly involving cannabis.

NSW Board of Studies president Tom Alegenouris said there has always been anecdotal evidence that some students seek desperate remedies.

"Our argument is that it actually works against you," he said. "You are not at your best, you are not at your clearest, you are not going to be able to show what you can do under artificial influences."

HSC drugs: What do they do? 

Ritalin

A central nervous system stimulant, Ritalin (methylphenidate), affects chemicals in the nerves and brain that contribute to hyperactivity and impulse control in patients with attention deficit disorders by increasing mental focus.

Modafinil

A wakefulness-promoting agent, Modafinil is used to treat excessive sleepiness caused by narcolepsy or shift work sleep disorder (sleepiness during scheduled waking hours). It has been anecdotally linked to enhancing cognitive abilities.

Melatonin

A hormone that is produced by the pineal gland in the brain, its production is increased at night to aid sleep and is reduced in bright light. It is taken artificially as a sedative through tablets to induce sleepiness.