There are a huge number of reasons this could be happening, but thankfully there’s also action you can take.
1. Your long-acting insulin could be the problem. I used to find Lantus really unpredictable, and nowhere near to lasting the 24 hours advertised. You could try changing to a different basal insulin and see if that helps. I’m not sure which one you use at the moment, but it might be worth having a look at what else is available.
2. The dawn phenomenon. For many of us, our blood glucose levels rise of their own accord on waking. I combat this by split-dosing my basal insulin, taking half at 10pm and half at 6.30am. Your times might need to be different but again, it could be worth discussing with your health team.
3. Insulin ‘stacking’. How long do you go between insulin doses? Do you snack a lot between meals and cover them with extra doses of short-acting? This can make blood sugar levels extremely unpredictable. Short-acting tends to work for around 5 hours in a lot of cases, and peaks about 1 hour after injecting (not sure if this is the case with newer insulin like Fiasp). If you’ve got various peaks happening, you’ll probably automatically take additional snacks to ward off hypos, causing both high and low blood sugars.
4. Delayed Stomach Emptying. Also known as Gastroparesis, this is where the stomach takes longer than usual to process the food it receives. It’s a diabetic complication caused by damage to the vagus nerve, and can make it difficult to match insulin to meals. There are treatments available, and at least one case where severe gastroparesis was apparently reversed by exemplary blood sugar control (Dr Richard Bernstein claims that after 14 years of following a low carbohydrate diet and keeping his levels in non-diabetic range, his digestive function became normal. Some people question this, but he’s a healthy type 1 diabetic now well into his 80s so he’s doing something right).
I find that when I pay close attention to what I’m eating and injecting, previously insurmountable problems become far more predictable. I know it’s a pain, but can you keep a food/insulin/blood sugar diary? It might reveal trends you were unaware of. It could also be a good idea to talk to your health team about the Freestyle Libre. Mine’s NHS funded and massively helps my control.