On days when we absolutely have to leave work at a specific time - for an appointment to see the doctor, say - we somehow manage to do it.
The key to a more productive working life, according to development coach and trainer Sue Camish, is to view every day in this way.
Ms Camish says: 'Work not only expands but also contracts to fill whatever time is allotted to it. Planning ahead enables you not only to do your work faster, it also helps you unwind and reduces stress.
'To-do lists are not very helpful, because they usually contain a month's worth of work. You finish the day feeling overwhelmed because most items haven't been ticked off.'
When you plan the next day, work out how much time should be devoted to meetings and other fixed appointments, checking
e-mails and other unanticipated interruptions, so you know what you've got left. Give yourself a reasonable number of work tasks to complete.
'It's a good idea to prioritise,' says Ms Camish. 'Save small jobs for breaks between meetings, after lunch, or whenever your energy is low. Ask yourself which jobs really need doing first.
'It can be very tempting to say, 'I'll just do that piece of work after lunch/after I've checked my e-mails/made that phone call'. It's a good idea to set fixed times during the day to deal with regular things, such as e-mail and post. That way, you won't put off more important tasks in favour of these.'
Learning to say 'no' is an important skill, she explains: 'If we're with a customer or in a meeting, we have no problem saying no. We have to learn that our own work time can be just as important. It's a good idea to fix a rota with a colleague, where they answer your phone for one hour each day, giving you time to catch up on things. You can do the same for them in return.'