Laying down naturally increases the blood flow to our kidneys which ends up in the production of more urine than when we are upright.
When we lay down to sleep at night time (only at night, not during day naps), our body produces a hormone called anti diuretic hormone (ADH). This hormone tells the kidneys to produce less urine than normal to allow us to have a more restful sleep without needing to keep going to the toilet.
The components in our body are kept to strict equilibriums using different methods. Even the slightest high or low of some components can be deadly. For example our pH is kept within strict limits using buffer systems, our calcium levels are regulated with the bones being a source and a store at the same time and our sodium levels are regulated using the kidneys and our hydration levels. If you consume too much salt (sodium chloride), you get thirsty to make you drink water to dilute the excess salt and the excess fluid then increases the volume of blood and hence increases your blood pressure.
The health risk of drinking too much water before bedtime comes from the dilution effect it has on your blood (risk of blood sodium getting too low) in the presence of ADH where the kidneys are stopped from being able to quickly get rid of the excess water. It does still end up creating too much urine which makes you go to the toilet but not as quickly as your body would otherwise get rid of the excess water to return to equilibrium.
This is why people can die from drinking too much water too quickly (like in radio station competitions).
The reason why people with sleep apnea need to urinate a lot through the night, is because they are waking up so often throughout the night that their body doesn't realise they are asleep and doesn't release ADH.
It's best to stay well hydrated throughout the day and stop drinking fluids a couple of hours before bed. Tiny sips should be ok.
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