A modular sleep system spreads the work across several pieces: insulation below you, a sleeping bag or bag combination around you, and weather protection outside. It is useful when conditions change across seasons or elevations.
Start with the sleeping mat. A warm bag cannot compensate for steady heat loss into the ground. Match the mat's insulation to the coldest surface temperature you expect, then choose the bag on its comfort range rather than its extreme figure.
For milder trips, a compact bag such as the Eagle, Tropen or Defence 1 may be enough. A Grizzly fleece liner can add comfort, help manage dirt and extend the range in suitable conditions. The Defence 4 covers colder use, while the Survival One is intended for more severe cold at the cost of extra volume.
Add a bivy only when the shelter plan requires it. A bivy improves protection from wind, damp ground and weather exposure, but it also changes ventilation and moisture management.
Test the combination before a serious trip. Make sure the layers fit without compressing the insulation, practise getting in and out, and confirm that the complete packed system fits your load plan. A system is valuable because its pieces can be changed; carry only what the actual conditions justify.