First I want to set some things straight.

It'd make sense to just fire around the clock.

1. Logistics and the limits of industry would make such a feat impossible. The western frontline was roughly 600 km's and stretched from the North Sea in Belgium all the way to Switzerland. For example, to make the massive 1917 push into Flanders possible the British already started improving and building local railroads in 1915 to ensure supplies would be adequate. Supplies then had to be stockpiled for the, 10 day,opening barrage with 3000 guns which shot 4.5 million shells.
2. The Western front was constantly adapting to artillery. Trenches, bunkers, dug out's etc were dug to offer some protection. Helmets were given to the soldiers to protect against debris and shrapnel.
3. Limits of equipment. A ton of different gun's were used but all of them have the same flaw in the end. Extensive use leads to deterioration of the barrel. All nations had to ramp up production massively while readjusting to the new economical restraints (war and blockades instead of free-trade). All sides, but especially the Germans, had to look for substitute materials, chemicals etc to manufacture their weaponry and ammunition.

So continuous massive bombardments without a direct purpose are wasteful and their effectiveness wasn't that great against dug-in soldiers. Barbed wire would remain in place, the landscape would be even more hellish making a push even more difficult.

But then you'd also need sleep and rest if you were to spend weeks at the front wouldn't you?

The trench system was massive, roughly 40.000km was dug in western Europe. When trench-warfare "stabilized" all major nations utilized a rotation-system. A prolonged stay in the first trench was inhumane. Now these rotation system changed depending on circumstances like weather, available reserves, which sector, the difference between countries etc etc and evolved throughout the war. Normally you would be 2 to 4 days in the first trench system, then rotate to the second trench system, then to the third and eventually reserve/leave/training/hinterland duty. Before starting all over again.

The shellings and artillery fire; was it constant from like the early morning until night time or did it only happen before an attack?

Random shellings happened off course, we are talking about a massive frontline that was in close contact for 4 years. But most of the times there was some purpose behind it; even if it's only calibrating the guns, measuring different firing ranges, to draw out enemy fire to localize their positions, etc.

Shelling what is assumed to be a forward observation, or preemptive fire against a supposed attack post might also look random.

The trenches were also a hive of activity, parts had to be rebuilt, new ones had to be dug, placing barbed wire or cutting it, establish listening and/or observation posts, etc etc. These things were mostly done at night to offer some protection against snipers, machine guns and artillery. Since smaller raids (to capture some prisoners for intel or disable some forward posts) were also conducted at night, the night was often the most dangerous part of trench life in a quiet sector.

To hinder these operations one might use it's artillery but due to the limited information a prolonged shelling at night is again not the most effective.

Now all of this is written from the perspective of the Western Front and mostly based on Belgian, British, German and French (in this order) sources.

The Western front was constantly adapting

The killing power of modern armies was unprecedented, what started as a mobile war aiming for a knockout punch turned into a war of constant attrition on a scale never seen before. The armies and the soldiers adapted. The first "trenches" were often already in place, think of roads, railroads, which had ditches on the sides, canals and rivers with their dikes or the agrarian countryside with farmlands separated by shrubbery and ditches. What were often only temporary, inadequate hideouts would evolve into the trenches. What were once shallow "straight" lines were depending on the terrain either raised to provide more cover or dug deeper. (This depends a lot on the water level, in west Flanders trenches had to be built with sandbags because digging in some areas would only make the water problem worse)

The "straight" lines would also be replaced by a labyrinth with many corners, dead ends, bunkers for men and material. The short turns were to protect against artillery strikes. In a straight line to many would be hit by the explosion and debris while being around the corner offers some protection against the worst of it. Enemy soldiers invading the trench would also be less familiar with the layout then the defending troops.

Changing tactics

Around the time of the Marne, in 1914, the mobile war shifted to trench warfare. We can roughly say that it started at the southern part of the front, moved to the center after the German defeat at the Marne on 12 September and moved further north during the Race for the Sea where for a month and a half both sides tried to force a breakthrough. The flooding of the Yserdelta by the Belgians, the British holding Ypres and the French supporting them both and finally the onset of winter with give both exhausted sides some reprieve.

The change for a knockout punch was lost and modern industrialized nations state would adapt to a war of mass production, destruction and attrition.

Both sides would fire and hire officers, change their equipment, ramp up their production and training capacities and adapt to trench warfare. The Germans, on average, would take a more defensive position. They had captured almost the entirety of Belgium, Northern France, both major industrial hubs. In their retreat after the Marne they had the first pick of terrain and settled most of the high ground.

Life in the trenches

Guard duty first and foremost, maintaining,restoring and improving the trenches second. Water was a major problem, especially in Flanders, so dugouts, bunkers, etc had to be pumped, often manually. Repairing damage to the trenches was a constant task and the trenches had to be extended, this could be with sandbags, tunneling, digging or a combination of these of course. With the rotation system soldiers had some reprieve of front-line duty.

Now to keep in simple, and to go back to artillery.

An army is an organization, artillery support has to be requested and authorized. The rank at which artillery support could be requested was decreased and communication with radio and telephone improved. This allowed both sides to more rapidly deploy fire support where needed.

Now disregarding prepared offensives for which massive amounts of ammunition were stockpiled and barrages were planned at specific timings to specific regions of the trenches artillery could be requested by an officer in the trenches when needed.

So mainly as a reaction to (supposed) enemy activity. One of the most opportune times would be during the enemies rotation of troops. Much more targets, much more movement. The same goes for when food is delivered, the enemy is preparing an offensive etc etc.

Source: I'm a Belgian WW1 tourguide and I've worked on several exhibitions about WW1.