Early birds.

Early morning flat water.

In "Ultimate Back Fitness and Performance", Stuart McGill notes that upon rising from bed, the intervertebral discs are at their greatest point of hydration. While sleeping horizontally, the pressure on the spinal column is removed, and fluid flows into the discs allowing the cells of both the annulus fibrosis and the nucleus pulposus of the discs to be nourished during rest. When the discs are fully hydrated in the morning, they are much more pliable than usual, thus highly susceptible to deformation generated by spinal flexion. This is the time of day that they are most vulnerable herniation injuries. (Don't we all know of someone who 'threw their back out' tying their shoelaces before leaving for work?) 

Fortunately this state doesn't last very long. Once we rise and begin to walk around, the intra-vertebral fluid is compressed out of the discs with loading on the spine. This process returns the discs to their regular, high performing state. Up to 90% of the fluid is removed during the first hour of the day.

McGill touches on the morning vulnerability of the discs several times in his book. In one section he answers the question of whether athletes should train right after rising from bed. The answer is clear. No. McGill recommends avoiding any spinal flexion (bending activities) during the first hour of the day, whether rehabilitative or performance enhancing in nature. Then, as if he wanted to make sure he got my attention, he uses rowers as his example! 

"Rowers pay dearly for their early-morning flat water; their backs would be much better served by training later in the day."

As a trainer and a coach, this is something that I must remember every morning I am working with someone. Early morning workouts are practically a gold standard in the fitness world, and as McGill correctly notes, rowers' love of flat water attracts them to dawn rowing sessions like moths to a flame. Additionally, there are many of us for whom first thing in the morning is our only option to hit the gym, and metabolically speaking, it is really a great time of day for physical activity. 

McGill is clear, contraindications within the first hour of the day include sit ups, crunches, spinal stretches, and there is no doubt rowing without proper preparation suddenly looks very risky. Thankfully, he also provides a sketch of a solution. Prior to an early morning row or workout which involves flexion, "arrange a progression to spine loading that produces progressive fluid loss in the disc to reduce bending stresses".

So we don't have to panic, we just have to use our heads.

1

If you are a gym rat, choose a warm up that can be done upright. Walking, running, or treadmill work are a great low impact way to load the spine. Avoid cycling; your spine will likely be in some form of flexion while seated. You can also do upper body calisthenics while standing to prepare shoulders for safe lifting.

2

Avoid exercises that require flexion (sit-ups, crunches) or that require strict form which, if compromised, would result in flexion (squats, deadlifts). Save these exercises for later in your workout when you are alert, capable of strict form, and have been loading your spine vertically.

3 

Whether you are a lifter or a rower, a great exercise to do before your workout would be the farmer's walk. You are upright (neutral extension of the spine), walking (the most therapeutic motion for the lumbar spine), and carrying weight (vertical compression of the discs).   

4

Rowers, once in the boat, make sure you warm up with light pressure, soft catches, and very strict back form. Then you can progress to your drill work, and then the row.

Time to hit it!

The time it took you to get ready, eat, and get down to the boathouse or the gym should have gotten you part of the way through your first hour. If you add in a warm up and the farmer's walk, you should be ready for your gym session. Rowers get to add in the time to prepare equipment and say good morning to everyone before they are dockside and ready to safely row.
 
Farmer's Walk

Grasp a dumbbell in each hand and let them hang by your sides. Walk. Pretty simple, but the added weight will increase the compressive load on your spine in a natural fashion. Walk across the weight room, your boat club apron, or the length of the boathouse. Do a series of round trips, progressing to a heavier dumbbell every 2 minutes. Walk for a total of 10 minutes.

References

McGill, Stuart. 2009. Ultimate Back Fitness and Performance: Fourth Edition. Waterloo, Ontario : Backfit Pro Inc. 
(pg. 52, 144).