If you thank your lucky stars every time you have an exam scheduled in the afternoon, because hello, extra cramming time, you should be cursing them instead - a new study has found that the later in the day an exam is scheduled, the bigger the decline in scores.

The study, which looked at 2 million standardised test scores from Danish school children aged between 8 and 15, found that for every hour after 8am that an exam was sat, test scores declined significantly, and the grades of students who were performing worst in class sunk the lowest.

The team, from the Danish National Centre for Social Research, found that for every hour later a test was held, scores declined by 0.9 percent - an amount equivalent to the effect of missing 10 days of school, New Scientist reports.

Interestingly, the study also found that a 20 or 30 minute break before an exam saw an average improvement of 1.7 percent in test scores - equivalent to if the exam was taken 2 hours earlier.

This suggests that if you don’t sit an exam right at the beginning of the day when your mind is fresh, you should be given a short break from class to prepare yourself mentally and combat cognitive fatigue.

"I’m very interested in what’s going on in these breaks," one of the researchers, Hans Henrik Sievertsen, told New Scientist. "Is it because they have something to eat, or fresh air? If we know that, we can maybe speculate why some children are more affected than others."

So students should be given their exams as soon as class starts at 8 or 9am? Not necessarily, because another study, published in 2010, found that when teenagers at Monkseaton High School in England were allowed to start school at 10am every day for two years, their exam scores skyrocketed.

Sally Weale reports for The Guardian:

"Following the experiment … GCSE results at Monkseaton went up from 34 percent of pupils scoring five A-C grades including English and maths, to 53 percent; the same results went up even more sharply for disadvantaged students, from 12 percent achieving five A-C with English and maths to 42 percent."

"It’s hugely more effective as an educational intervention in terms of raising achievement and health than any alternative," lead researcher Paul Kelley from Oxford University’s Sleep and Circadian Neuroscience Institute said at the time. "I should have done it sooner. Nothing I had ever done in all my teaching made such a difference."

Kelley and his team are now in the process of a follow-up experiment involving 106 schools and almost 32,000 teenagers in the UK to see if this 10am benefit is real.

So in a perfect world, students would get a sleep-in and sit their tests first thing in the mid-morning. But in the not-so-perfect real world, you start school when your school tells you to, so if you have a choice in when you sit your exams, make sure you pick the early slot, or demand a break beforehand. Cite "cognitive fatigue" - we've got your back.

The Danish study has been accepted for publication in Proceedings of the National Academy of Sciences.