Your bag doesn’t need a complete wash every time; for it, a little TLC is enough, not a full-blown bath. Because constantly washing a sleeping bag puts it through to wear and tear and also decreases the loft. Spot cleaning or sponging is your first line of defense. Make a paste of a little non-detergent soap and water, and use a cotton bud or toothbrush to clean the spot gently.
Keep your focus on the hood and collar where hair and skin oils tend to gather. By holding the spot or liner fabric away from the insulation, you can also wash and rinse the area without wetting the whole tent.