No, you're not imagining it.

When we were younger, it seemed like the school year lasted forever - and summer break lasted a week. And now? It feels like we were just ringing in 2018 yesterday - and yet, it's actually August. If you feel like time is flying by as you get older, don't worry: you're not imagining it. There's a reason why that happens - and, unfortunately, it has nothing to do with having fun. Dr Christian Yates, a mathematical biology lecturer at University of Bath, explained in The Conversation why it feels like the days, weeks and months seem to fly by as you age. It's all down to three scientific theories.

We get comfortable

The first one is that we get more familiar and comfortable with our surroundings as we get older. "We don’t notice the detailed environments of our homes and workplaces," Dr Yates added. Basically, the more comfortable we get with our day-to-day surroundings and life experiences (like going to work), the faster time seems to run. But for kids, the "world is an often unfamiliar place filled with new experiences to engage with" - so the period of time feels much longer. Dr Yates explained that, according to the theory, this is why time runs slower for children than adults who are stuck in a routine, as they are using more brain power. He continued:

"The more familiar we become with the day-to-day experiences of life, the faster time seems to run, and generally, this familiarity increases with age."

We...well, get older

The second theory involves our age - and how our metabolisms slow as the years go on. This ends up making your internal body clock feel like its actually going faster. Meanwhile, your heartbeat and creating rate - biological pacemakers - both slow down as you get older. Dr Yates wrote:

"Children’s biological pacemakers beat more quickly, meaning that they experience more (heartbeats and  breaths) in a fixed period of time, making it feel like more time has passed."

How long we have lived

A third theory says that the way the perceive time is actually proportionate to how long we have lived. For example, a two-year-old waiting for their birthday is going to feel like it takes an extraordinarily long amount of time - after all, a year is half their life. But for a 10-year-old, it's only 10 per cent of their life - and for a 20-year-old, it's only 5 per cent. So it feels like your birthday is back around again before you can even blink. According to Dr Yates, a two-year-old waiting a year for their birthday is proportionate to a 20-year-old waiting to turn 30 years old. He said:

"I don’t wish to end on a depressing note, but the five-year period you experienced between the ages of five and ten could feel just as long as the period between the ages of 40 and 80. "So get busy. Time flies, whether you’re having fun or not. And it’s flying faster and faster every day."