There was a time when 'nine to five' was the normal working day. There were people like nurses, doctors, the emergency services and those working in continuous manufacturing jobs who had to work shifts but they were in the minority.
However, these irregular working patterns are becoming more normal in this technological age. Digital industries, for example, need people at their desks throughout the day and night to update sites or process requests.
And our demand for round-the-clock entertainment and television or radio programmes means more people are required to work throughout the night.
But now scientists at the Sleep Research Centre at the University of Surrey say that not only does irregular shift work have a similar effect to severe jet lag or repeatedly missing sleep, it has a damaging impact right down to the level of our DNA.
They even warn that prolonged shift work can have long-term health consequences such as increasing the risk of cancer or diabetes.
We asked four shift workers about how their job affects them and how they make it through the night.
The radio presenter: 'The hours don't bother me'
Mairead Campbell (24), lives in Belfast. She is a DJ for Cool FM and presents her show between 2-6am. She says:
I've been presenting the show for two and a half years but I think you never really get used to working anti-social hours. The nature of my job means I might have to do things – like interviews or a radio packages – during the day as well, and I also provide cover for other slots.
I started these hours in summer 2011. Cool FM has a very large number of listeners in Northern Ireland, so I knew that even in the small hours we would still have a big audience. I knew what I was I was in for but I was amazed at the number of people that are up and about at that time and the real need for entertainment like a radio show. It's helped me understand that Northern Ireland is a 24-hour society.
We get all sorts of listeners, from those working in travel or leisure industries, and those who make night-time deliveries, to students who are studying all night or have been out partying.
You get people who can't sleep, as well as airport staff, postmen and retail workers who work through the night. The hours I work – four a night – are wee buns compared to what people across the province have to work.
I obviously have to do quite a lot of talking and have to remain alert the whole way through the show. There are nights that I've come in, having worked since noon the previous day, and listeners will text empathising with me. I've learned different techniques to get me through.
My sleep is erratic. I go to bed after the 6am show for about four hours, then I try and get a few more hours at some point in the afternoon, but I'm not always able to.
Initially, I became hooked on energy drinks and fizzy drinks for the sugar rush and the caffeine. I also snacked, which was a bad idea. Other than the health risks, getting home at 6am after a night full of energy drinks means you'll spend hours tossing and turning. I was eating at ridiculous hours and eating convenience foods or takeaways, too. I think that generally made me feel worse rather than better. I felt groggy, and heavy, and not awake when I ate like that.
Nowadays I don't eat breakfast – I don't need energy in the morning as I'm generally asleep. I have a light lunch when I get up and then dinner later on. Before my show starts at 2am I'll bring in soup or sandwiches to nibble on. I'm only really getting a hang of my diet in the last six months.
It's been one of the most challenging aspects of my job as you really need the energy. Now, I drink lots of water all the time instead of fizzy drinks.
I get to socialise at the weekends. Most of my friends work regular hours. I started working in the media at the age of 16, and I've always gone to events and club nights. A social life is something I've been happy to sacrifice. I love my job and I've always wanted to do it. I do go out though.
I think, as time goes on, I would love to come off nightshift and progress to an earlier slot but that's more to do with career progression rather than the hours I work."
The nurse: 'I will only work two nights in a row'
Pamela Hughes (61), is an agency staff nurse. She lives in Islandmagee with her husband John and they have two grown-up sons, Christopher (32), Peter (30). She says:
I've been nursing for nearly 40 years. I worked for Marie Curie for 17 years and even spent some time as a nurse manager. Apart from a few years in management I have always worked shift work.
Nowadays as an agency nurse I work at least three 12-hour shifts a week, normally in nursing homes. As my husband John has retired we have more time to spend together and I can pick and choose when I work. Working for an agency means I can take a holiday for a bit longer instead of having to worry about annual leave.
I like to work either night shifts or day shifts in a week, but not both. I also don't like working more that two night shifts in a row as it can get a bit tiring.
I started working nights a long time ago, when the boys were young – I could go to bed then collect them from school and be here until my husband came home to take over.
I don't mind if I'm working nights or days. When you've done this job for this long you know what to expect. You work in different places each time but you're told a lot about where you're going before you start, such as where it is and how many residents there are.
If I'm working nights I try and sleep in the afternoon for a couple of hours. I'll have dinner then I'll take some water and a couple of sandwiches to nibble on during the night. You always need to eat when you work at night. You're on the go checking every single person every hour so you need to keep yourself fed. I also have 'supper' of a couple of ham sandwiches when I get home from night duty, but not breakfast. You feel a bit upside down, but it feels like a normal day at work to me now.
When I get home from two nights in a row I get up after lunch time the next day, go to bed at a normal time that night then I'm fine again. I don't think everyone can do it. I've heard nurses say that they can't do nights, that they make them sick. Some people aren't night-time people.
I do drink plenty of coffee. I have one before I drive home to keep myself awake, usually with the window open and the radio on as well.
Sleep before a nightshift is really important as is having dinner before and a snack for during the night. I think you should walk around too to keep yourself awake and to help pass the time. That keeps you alert."
The hotel manager: 'My shifts let me spend time with son'
Michael Sullivan (34), is the assistant general manager of the Merchant Hotel in Belfast. He lives in Glengormley with wife Kathryn and they have a son, Sean (three months). He says:
I started working in hotels in Dublin in 1998 so I've done shift work throughout my career. Sometimes the shifts go on until midnight and sometimes they can go on throughout the night.
Now I generally work three days during the week when I start at 10am and go on until 6 or 7pm. Then at weekends I can start at about 6pm then go on until 3 or 4am.
We have a lot of functions, particularly weddings at weekends and I'll be there in a duty manager role. What time I finish depends on business needs.
During the week I'm mostly office-based. The shifts actually work in my favour at the moment. I tend to take a Wednesday and a Sunday off, so it can be beneficial for childcare and spending time with my son.
At weekends it doesn't have much of an impact. I start work at 6pm on a Saturday which means I have a whole day off before I go into work.
I find it quite easy to find a balance between work and home demands. There are days I don't get much sleep, but I put that down to having a new baby. If I'm working at night then I try to go to bed for a couple of hours during the day. It is important that you get the proper rest before you start.
When I'm on a late shift I don't go near caffeine or coffee. The last thing you need when you get home at 3am, is lying awake because you've been drinking coffee all night. I stick to water most of the time.
I don't eat that much more. My meal times are the same as they would be if I was working nine to five. I'm on the go the whole time on a night shift and I think that every shift is unique.
No day is ever the same, which makes it interesting and more enjoyable. If you're going to work these kind of shifts you need to enjoy them before you commit to doing it."
The warehouse worker: 'I focus on the positives'
Lukasz Ucinski (33), lives in Belfast with his girlfriend Magdalena. He works as a warehouse co-ordinator for Hendersons. He says:
I've been working at Hendersons for more than five years. My shift pattern is regular, 3pm until midnight from Monday to Thursday, then 1pm until 5pm on Friday. Those are my contract hours but due to operational requirements I often find myself staying longer. The latest would be 6am but I'm more likely to be at work until one or two in the morning. It varies.
I haven't always done this shift. I started off working from 6am until 2pm until I got this promotion. I accepted it and I find it's something I've got used to.
It all depends on your attitude and the way you approach it. If you focus on the pros, not the cons, then you'll be fine. There are no traffic problems, no queues at bank machines or in the all-night shops.
I get to see Magdalena on Friday, Saturday and Sunday. I actually think it's better that way as you get the time to miss someone.
I know some people have problems with night shifts, they can't sleep or eat properly, but I think it depends on your approach.
I go straight to bed when I get home. I used to watch TV for a while but not any more. I can then get up at 9am and that gives me plenty of time for anything I need to do during the day. I try to sleep for at least seven hours which I think is enough.
I don't eat breakfast at all so my first meal is at lunchtime, then I would have a meal at work and that might be a bit later than most people. I bring food with me to work and on Thursday nights we have a take-away night where we'll order pizza or a Chinese meal.
I drink coffee in the mornings. We're pretty busy here so adrenaline is enough to keep me going through the shift in the evenings.
If my circumstances change in the future I think I'll still keep working shifts. The way I see it if there are children in my future then I can look after them in the mornings.
If I ever have to start working regular hours then I will try to find the positives of working those kind of hours too."