Circadian rhythms are what regulate your body wake/sleep cycle: they pretty much go in automatic, and tell your body when it is time to fall asleep and when it is time to be awake.
The mechanism that regulates them is actually quite complex, and involves a couple of very small glands in the brain and a cascade of hormones, but to keep it simple I will focus on the two main hormones: melatonin and cortisol.
Melatonin is the hormone that your body secrete when is dark and that makes you fall asleep, while cortisol is the hormone of stress and alertness. We can say that this hormones are mutually exclusive meaning that much of one, inhibit the production of the other and vice-versa
If you have trouble falling to sleep at night, and you feel drowsy and sleepy during the day, and wide awake and full of energy in the evening, it might mean that you production of these two hormones is out of sync and you need to rebalance it.
The problem with this is that if you are feeling drowsy and lacking energy during the day, you are much more likely to rely on coffee and sugar to keep you going, which in turn can have a negative ripple effect throughout the rest of your day, making you grumpy and more reliant on carbs, stimulants and nutrients poor foods, sabotaging your efforts to lose weight or to have an healthy diet.
Circadian rhythms are mainly largely regulated by daylight: too little daylight during the day (especially morning hours ) and too much artificial lights or blue lights during the evening, can easily mess up you wake/sleep cycle.
HOW TO NATURALLY REGULATE YOUR CIRCADIAN RHYTHMS
1. Try to maintain the same sleep/wake timing.
Trying to go to bed and wake up at around the same time every day will help create a pattern that will make it easier for the body to start the production of melatonin at the right timing for you.
2. Get plenty of daylight.
Daylight is what ultimately regulate your wake/sleep cycle, so getting plenty of it, especially in the morning hours, will tell the body that is awake and its to stop producing melatonin.
3. Dim the lights 1-2 hours before going to sleep
Dimming the light in the evening will tell the body that its time to go to sleep, and to start producing melatonin, so that when you finally hit the pillow, you can easily fall asleep.
4. Switch off your computer and phone at least 1 hour before bed.
Electronic devices like computers or phones have what is called blue light, which can mess up the natural body understanding of whether is day or night. Simply switch them off an hour before going bed.
5. Try doing something relaxing in the evening.
If cortisol is high in the evening, trying to do something relaxing can help. The choice is yours: from a calming bath (maybe with some magnesium rich salts) to some stretching or yoga, or even a nice movie on the telly, if that helps. Likewise try to avoid activities that can wake you up, like too much reasoning before bed, or arguing with your partner.
6. Mentally switch off
Many people can be kept awake because they are trying to organize and remember the chores and activities of the following day. The more you are stressed, tired and overwhelmed and the more you are more likely to be afraid to forget. But this sense of urgency and overthinking is keeping your brain switch on, basically telling you body that it needs to be awake. Try to be smart with your do to lists, and try to mentally switch off: reading a book, some stretching or meditation can be helpful.