Night Serums

Skin does most of its repair work overnight, so in fact to get greatest benefit, retinol skincare products are better used at night time instead of the morning. Retinol's potency and effectiveness can also be degraded by sunlight if applied in your morning routine. Using at night also allows you to follow with a heavier moisturising or night cream afterwards to minimise any dryness from the retinol. Such creams commonly contain comforting ingredients like hyaluronic acid.
Retinol promotes skin cell renewal – a process that naturally slows as we age. Boosting this process helps replace dead skin cells on the surface with fresh, new ones − beneficial for blemish-prone skin, improving skin texture, and pigmentation concerns.
And it's an antioxidant, reducing oxidative stress and promoting the skin's natural collagen production, helping to delay onset of visible signs of ageing such as fine lines, wrinkles, or uneven skin tone.
Lastly, because using retinol increases sun sensitivity, a broad spectrum, high SPF sunscreen is recommended during the day.
Learn more about using retinol HERE.

The answer to this will depend upon your specific skin concerns, but often a night cream and hydrating serum are equally important for your skin.
Night creams tend to be quite thick, penetrate slowly, and effective mainly on upper layers of the skin, whereas hydrating night serums are lightweight and formulated to penetrate quickly and more deeply.
A serum is usually focussed on delivering a high concentration of a single ingredient (such as hyaluronic acid in a hydrating serum), so it's the most effective at targeting a specific skin concern. A night cream usually contains a blend of different ingredients at lower concentrations, making it a more general-purpose product to suit various skin types and needs in addition to keeping skin hydrated.
If you use both, apply the hydrating serum before your moisturiser or night cream.

Some ingredients, such as hyaluronic acid or niacinamide (vitamin B3), can be applied in serum format as part of either (or both!) your morning or nighttime routines.
Conversely some (especially anti-ageing serums containing antioxidants like vitamin C, resveratrol, silymarin, or ferulic acid are recommended for use in the morning, to best protect the skin from oxidative stress generated during the day by the sun's rays and other environmental aggressors.
However some ingredients are best used only at nighttime, including retinol. That's because retinol boosts skin cell turnover and collagen production, both most useful while you're asleep, when the skin undergoes repair and renewal. Sunlight can also reduce the potency of retinol.
Chemical exfoliants like salicylic acid, glycolic acid, and lactic acid make the skin more sensitive to sunlight, so a facial serum containing any of these is also recommended for nighttime use only.

That will depend on the active ingredient in the facial serum.
Some, such as retinol, which boosts both skin cell turnover and collagen production, are certainly more useful to the skin at night, when it's undergoing repair and renewal. Sunlight can also reduce the potency of retinol if applied in the morning.
Other serums, especially anti-ageing serums containing antioxidants like vitamin C, resveratrol, or ferulic acid, are better applied in the morning so they can protect the skin from oxidative stress generated by the sun's rays and other external aggressors during the day.
And in some cases it might depend on what other active ingredients you are using in your routine. For example, salicylic acid and niacinamide are at their most effective at quite different pH (acidity) levels. Salicylic acid serum is formulated with an acidic pH between 3 and 4, but niacinamide works best at a more neutral pH of around 6. Using both at the same time of day compromises their penetration ability and performance, so it makes sense to split them between your morning and evening routines (in this example we'd recommend niacinamide in the morning and salicylic acid at night).

There's no reason you can't apply a vitamin C serum at nighttime. However...
In addition to skin brightening, vitamin C's anti-ageing powers include powerful antioxidant protection against environmental aggressors. Because of that, SkinCeuticals normally recommends applying a vitamin C facial serum in the morning, to take advantage of the environmental protection it provides the skin through the day, when UV radiation is at its highest.
So the answer could depend on what other skin concerns you're targeting in addition to dull, tired-looking skin. Compatibility with other actives in your skincare regimen may also be a factor to consider.
A facial serum is the most effective way of using vitamin C topically. Vitamin C is suitable for all skin types, including dry, oily skin, or sensitive skin.

That depends on what skin concerns you want to correct, but you'll often find a night cream and night serum are equally important for different purposes.
A serum is lightweight, formulated to penetrate quickly and more deeply. Intended to deliver a high concentration of a single ingredient, it's the most effective at targeting a specific skin concern.
A night cream tends to be quite rich and thick to combat moisture loss that occurs overnight. They penetrate slowly and are effective mainly on upper layers of the skin. Whilst primarily intended for intense hydration, night creams usually contain a blend of different ingredients at lower concentrations, making them a more general-purpose product to suit various skin types and needs.
If you use both, apply the night serum before your night cream.