About 500,000 tickets are expected to be sold in NSW and the ACT today. NSW Lotteries expects queues to start forming at many outlets from lunchtime. By the time of the draw, about 1 million people in NSW and ACT will have tickets to Oz Lotto's second-biggest jackpot of the year.

Many of those ticket-holders were already dreaming about how they would spend the money, said Darren Christensen, an expert in decision-making and problem gambling.

"When you are close to a positive outcome the anticipation, the arousal builds. Tonight you find out if you are a millionaire – it's very persuasive and exciting," he said.

"You're thinking about winning the money – whether it's spending the money or just the experience of winning something would be quite overpowering for many people, especially people with problems with impulse control or people who make decisions more on emotional reasons rather than by calculating things."

Typically, such dreams flitted between paying off debts, taking a holiday to an exotic location or buying gifts for yourself and your loved ones, said Dr Christensen, of the University of Melbourne. An older winner might dream about paying the mortgage and buying a house for the kids.

A Sydney man who won $24.5 million in Oz Lotto's $70 million draw in May said he would use the money to help his family. "This win means myself and my family will never have any money worries again," he said at the time. "I feel like I'm in a dream."

Winning such a sum will only ever be a dream for the vast majority of lottery players. Dr Christensen, a non-gambler, said that on any objective calculation "there is no good reason" to buy a ticket in the draw. "You're losing money, basically. The expected value is less than the amount you pay for the ticket."

Media coverage of tonight's draw – including this article – built anticipation and excitement among many people. "You don't think so much about the expected chance of winning; you're emotional about the possibility of winning. You feel energised and activated by being close to the positive outcome."

Of course, you are not very close at all to that positive outcome. Mathematicians have calculated the chances of winning the jackpot with a single ticket in a $50 million Oz Lotto draw as one in 45 million. In other words, a ticket-holder has more than a 99.99997796 per cent chance of not winning.

But most lottery players concentrate on the amount that could be won rather than the probability of doing so.

Gambling studies professor Mark Griffiths, of the International Gaming Research Unit at Nottingham Trent University, recently wrote that we tend to overestimate positive outcomes and underestimate negative ones. "If someone is told they have a one in 14 million chance of being killed on any particular Saturday night, they would hardly give it a second thought because the chances of anything untoward happening are infinitesimal.

"However, given the same probability of winning the National Lottery and people suddenly become over-optimistic."

One study, he wrote, found 22 per cent of people thought that if they played the national lottery every week until they died, they would win the jackpot at some point in their lifetime.

Lotteries also cash in on the "psychology of entrapment" with people who feel locked into choosing the same numbers each week. They keep playing for fear that as soon as they stop, their numbers might come up.

People buy lottery tickets in big draws because they do not want to feel left out, according to US psychologist Stephen Goldbart. The dream of winning lotto also increased in times of financial uncertainty, he wrote in Psychology Today. "When times are tough, and we are continually struggling to get in command of our financial lives, a little fantasy goes a long way to feeling better."

But how much better do lottery winners feel?

Call it schadenfreude but perhaps there is some consolation in the fact most lottery winners are only briefly euphoric before settling back to their "normal" level of happiness or unhappiness. A 1978 study titled Lottery winners and accident victims: Is happiness relative? found lottery winners were "not in general happier" than the control group.

Studies show each person has a relatively constant state of happiness, which spikes up and down depending on circumstances. Dr Griffiths believed a typically unhappy person who won the lottery, for example, would likely be happy for a couple of months before resorting back to their normal state of discontent.

The daily lives of most lottery winners improve, of course. But some find instant wealth also brings problems such as family tensions over money and feelings of isolation. Of course, that's something the vast majority of us will never have the pleasure of dealing with.