The science is easy on this one so read carefully and I'll keep it short.
The fact is, the sun is a major component of how well we sleep.
We have a body clock which controls our sleep (circadian rhythm) and lots of other major processes but lets focus on sleep.
Ideally, we wake in the morning and our body clock fully resets and allows us to get on with our day. However, this doesn't always happen fully. A major component of the reset is daylight - the bright light in the morning is absorbed by the eyes which sends a clear signal to the brain that its morning and resets the 'body clock' in anticipation that in roughly 16 hours the brain will prepare for sleep. Now, if we stay indoors for the first few hours of the day this signal is never fully sent by the eyes to the brain and creates a 'confused' body clock which results in a mismatch of when to be awake and when to be asleep resulting in you either sat up in bed trying to sleep or not sleeping well and waking up every few hours that night.
What can you do about it?
Luckily this is very easy to solve. You only need about 15-30 minutes of outdoor daylight in the morning to help with the body clock reset. Additionally getting daylight exposure (don't worry if its cloudy as the daylight still gets through and has a strong impact - UK weather!) throughout the day only solidifies this clarification for the body clock - the more light during the day the better.
Obviously as I'm sure you've heard me say before, phones, TV's and household lights have a similar effect on the brain so I recommend dimming the lights in the hours leading up to bed-time OR wearing blue-light blocking glasses which block out this light from being absorbed by the eyes and limiting time spent on the phone and watching Netflix! - we don't want to confuse the body clock!
To summarize -
Get daylight early in the morning AND throughout the day if possible. Then reduce light exposure in the evening as much as you can - this will set you up for a good nights sleep and allow your brain to know when its time to sleep.
FYI - Melatonin is the sleep hormone and this is largely regulated by light. If there's lots of light, Melatonin stays at low levels (day-time) in the brain. When it's getting dark, Melatonin increases in the brain signaling a soon to be bed time - allowing good quality sleep and a body-clock in sync with you - SIMPLE!